Transparent waveguide display

ABSTRACT

One embodiment provides an apparatus for displaying an image comprising: a first optical substrate comprising at least one waveguide layer configured to propagate light in a first direction, wherein the at least one waveguide layer of the first optical substrate comprises at least one grating lamina configured to extract the light from the first substrate along the first direction; and a second optical substrate comprising at least one waveguide layer configured to propagate the light in a second direction, wherein the at least one waveguide layer of the second optical substrate comprises at least one grating lumina configured to extract light from the second substrate along the second direction, wherein the at least one grating lamina of at least one of the first and second optical substrates comprises an SBG in a passive mode.

RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.17/153,588, filed Jan. 20, 2021, which is a continuation of U.S. patentapplication Ser. No. 15/943,590, filed Apr. 2, 2018, which is acontinuation of U.S. patent application Ser. No. 14/044,676, filed Oct.2, 2013, which is a continuation-in-part application of U.S. patentapplication Ser. No. 13/844,456, filed Mar. 15, 2013, which claimspriority to U.S. Provisional Patent Application No. 61/796,632, filedNov. 16, 2012, and 61/849,853, filed Feb. 4, 2013, the disclosures ofwhich are hereby incorporated by reference in their entireties.

There is a need for a compact transparent data display capable ofdisplaying image content ranging from symbols and alphanumeric arrays tohigh-resolution pixelated images. Examples of transparent displaysinclude HMDs, HUDs, HDDs and others. One important factor in each caseis that the display should be highly transparent and the displayed imagecontent should be clearly visible when superimposed over a brightbackground scene. The display should provide full color with an enhancedcolor gamut for optimal data visibility and impact—although monochromewill suffice in many applications. One important factor for HelmetMounted Displays is that the display should be easy to attach tostandard helmets or replicas thereof designed for training. The eyerelief and pupil should be big enough to avoid image loss during headmovement even for demanding military and sports activities. The imagegenerator should be compact, solid state and have low power consumption.In automotive applications the ergonomic demands are equally challengingand aesthetic considerations make yet further demands on the form factorof the display, which ideally should be capable of being hidden within adashboard when not in use. There is a growing need for more compact,cheaper and more efficient designs in many other application areas. Theinventors note the growing demand for HUDs in airliners and smallaircraft. Car manufactures are also looking to provide HUDs and HDDs intheir future models. The systems described herein may be applicable to ahelmet mounted head worn display for use in Augmented Immersive TeamTraining (AITT), essentially a live simulated training system forobserver training that augments or replaces indirect fires and aircraftsorties needed to certify or sustain observer skills.

The above goals are not achieved by current technology. Current designsonly manage to deliver see-through, adequate pupils, eye relief andfield of view and high brightness simultaneously at the expense ofcumbersome form factors. In many helmet-mounted display designs, weightis distributed in the worst possible place, in front of the eye. Themost common approach to providing see-through displays relies onreflective or diffractive visors illuminated by off axis light.Microdisplays, which provide high-resolution image generators in tinyflat panels, do not necessarily help with miniaturization because theneed for very high magnifications inevitably results in large diameteroptics. The ideal transparent display is one that: firstly, preservessituational awareness by offering a panoramic see-through view with hightransparency; and secondly, provides high-resolution, wide-field-of-viewimagery. Such a system should also be unobtrusive; that is, compact,light-weight, and comfortable, where comfort comes from having agenerous exit pupil and eye motion box/exit pupil (>15 mm), adequate eyerelief (≥25 mm), ergonomic center of mass, focus at infinity, andcompatibility with protective head gear. Current and future conventionalrefractive optics cannot satisfy this suite of demands. Other importantdiscriminators include: full color capability, field of view, pixelresolution, see-throughness (transparency), luminance, dynamic grayscaleand power consumption levels. Even after years of highly competitivedevelopment, head-mounted displays based on refractive optics exhibitlimited fields of view and are not adequately compact, light-weight, orcomfortable.

Displays based on waveguide technology substrate guided displays havedemonstrated the capability of meeting many of these basic demands. Theconcept has been around for well over a decade. Of particular relevanceis a U.S. Pat. No. 5,856,842 awarded to Kaiser Optical Systems Inc. in1999 which teaches how light can be coupled into a waveguide byemploying a diffractive element at the input and coupled out of the samewaveguide by employing a second diffractive element at the output.According to U.S. Pat. No. 5,856,842, the light incident on thewaveguide needs to be collimated in order to maintain its image contentas it propagates along the waveguide. That is, the light must becollimated before it enters the waveguide. This can be accomplished in avariety of ways and is not a concern here. With this design approach,light leaving the waveguide will be naturally collimated, which is thecondition needed to make the imagery appear focused at infinity. Lightpropagates along a waveguide only over a limited range of internalangles. Light propagating parallel to the surface will (by definition)travel along the waveguide without bouncing. Light not propagatingparallel to the surface will travel along the waveguide bouncing backand forth between the surfaces, provided the angle of incidence withrespect to the surface normal is greater than some critical angle. ForBK-7 glass, this critical angle is approximately 42 degrees. This can belowered slightly by using a reflective coating (but this unfortunatelydiminishes the see-through performance of the substrate) or by using ahigher-index material. Regardless, the range of internal angles overwhich light will propagate along the waveguide does not varysignificantly. Thus, for glass, the maximum range of internal angles is≤50 degrees. This translates into a range of angles exiting thewaveguide (i.e., angles in air) smaller than 40 degrees and generallyloss, when other design factors are taken into account. To date,Substrate Guided Optics (SGO) technology has not gained wide-spreadacceptance. This is largely due to the fact that waveguide optics can beused to expand the exit pupil but they cannot be used to expand thefield of view or improve the digital resolution. That is, the underlyingphysics, which constrains the range of internal angles that can undergoTIR within the waveguide, limits the achievable field of view withwaveguide optics to at most 40° and the achievable digital resolution tothat of the associated imager. Nevertheless, the lure of a compact,light-weight HMD based on waveguide optics continues to inspireinterest. One way to create a much larger field of view is to parse itinto a set of smaller fields of view (each compatible with the opticallimitations of the waveguide) and to time sequentially display themrapidly enough that the eye perceives them as a unified wide-angledisplay. One way to do this is with holographic elements that can besequentially switched on and off very rapidly such as a Switchable BraggGrating (SBG).

The optical design benefits of diffractive optical elements (DOEs) arewell known including unique and efficient form factors and the abilityto encode complex optical functions such as optical power and diffusioninto thin layers. Bragg gratings (also commonly termed volume phasegrating or holograms), which offer the highest diffraction efficiencies,have been widely used in devices such as Head Up Displays (HUDs). Animportant class of Bragg grating devices is known as a Switchable BraggGrating (SBG). An SBG is a diffractive device formed by recording avolume phase grating, or hologram, in a polymer dispersed liquid crystal(PDLC) mixture. Typically, SBG devices are fabricated by first placing athin film of a mixture of photopolymerizable monomers and liquid crystalmaterial between parallel glass plates or substrates. Technique, formaking and filling glass cells are well known in the liquid crystaldisplay industry. One or both glass substrates support electrodes,typically transparent indium tin oxide films, for applying an electricfield across the PDLC layer. Other types of transparent conductivecoating may also be used. A volume phase grating is then recorded byilluminating the liquid material with two mutually coherent laser beams,which interfere to form the desired grating structure. During therecording process, the monomers polymerize and the holographicpolymer-dispersed liquid crystals (HPDLC) mixture undergoes a phaseseparation, creating regions densely populated by liquid crystalmicro-droplets, interspersed with regions of clear polymer. Thealternating liquid crystal-rich and liquid crystal-depleted regions formthe fringe planes of the grating. The resulting volume phase grating canexhibit very high diffraction efficiency, which may be controlled by themagnitude of the electric field applied across the PDLC layer. When anelectric field is applied to the hologram (e.g., a suitably optimizedhologram) via transparent electrodes, the natural orientation of the LCdroplets is changed causing the refractive index modulation of thefringes to reduce and the hologram diffraction efficiency to drop tovery low levels. Note that the diffraction efficiency of the device canbe adjusted, by means of the applied voltage, over a continuous rangefrom near 100% efficiency with no voltage applied to almost zeroefficiency with a sufficiently high voltage applied. SBGs may be used toprovide transmission or reflection gratings for free space applications.SBGs may be implemented as waveguide devices in which the HPDLC formseither the waveguide core or an evanescently coupled layer in proximityto the waveguide. In one particular configuration to be referred to hereas Substrate Guided Optics (SGO), the parallel glass plates used to formthe HPDLC cell provide a total internal reflection (TIR) light guidingstructure. Light is “coupled” out of the SBG when the switchable gratingdiffracts the light at an angle beyond the TIR condition. SGOs arecurrently of interest in a range of display and sensor applications.Although much of the earlier work on HPDLC has been directed atreflection holograms, transmission devices are proving to be much moreversatile as optical system building blocks.

Typically, the HPDLC used in SBGs comprise liquid crystal (LC),monomers, photoinitiator dyes, and coinitiators. The mixture frequentlyincludes a surfactant. The patent and scientific literature containsmany examples of material systems and processes that may be used tofabricate SBGs. Two fundamental patents are: U.S. Pat. No. 5,942,157 bySutherland, and U.S. Pat. No. 5,751,452 by Tanaka et al. Both filingsdescribe monomer and liquid crystal material combinations suitable forfabricating SBG devices.

One of the known attributes of transmission SBGs is that the LCmolecules tend to align normal to the grating fringe planes. The effectof the LC molecule alignment is that transmission SBGs efficientlydiffract P polarized light (i.e., light with the polarization vector inthe plane of incidence), but have nearly zero diffraction efficiency forS polarized light (i.e., light with the polarization vector normal tothe plane of incidence. Transmission SBGs may not be used atnear-grazing incidence as the diffraction efficiency of any grating forP polarization falls to zero when the included angle between theincident and reflected light is small. A glass light guide in air willpropagate light by total internal reflection if the internal incidenceangle is greater than about 42 degrees. Thus the invention may beimplemented using transmission SBGs if the internal incidence angles arein the range of 42 to about 70 degrees, in which case the lightextracted from the light guide by the gratings will be predominantlyp-polarized. Normally SBGs diffract when no voltage is applied and areswitching into their optically passive state when a voltage is appliedat other times. However, SBGs can be designed to operate in reverse modesuch that they diffract when a voltage is applied and remain opticallypassive at all other times. Methods for fabricating reverse mode SBGsare disclosed in U.S. Provisional Patent Application No. 61/573,066,with filing date 24 Aug. 2012, by the present inventors entitledIMPROVEMENTS TO HOLOGRAPHIC POLYMER DISPERSED LIQUID CRYSTAL MATERIALSAND DEVICES. The same reference also discloses how SBGs may befabricated using flexible plastic substrates to provide the benefits ofimproved ruggedness, reduced weight and safety in near eye applications.

In a prior filing the inventors have disclosed a waveguide (SGO) displaythat produces a large field of view by parsing it into a set of smallerfields of view (each compatible with the optical limitations of thewaveguide) and to time sequentially display them so fast that the eyeperceives them as a unified image. This process is sometimes referred toas field of view tiling. One way to do this is with holographic elementsthat can be sequentially switched on and off very rapidly. In an earlierPCT Application No.: PCT/GB2010/000835, with International Filing date26 Apr. 2010, by the present inventors entitled COMPACT HOLOGRAPHIC EDGEILLUMINATED EYEGLASS DISPLAY which is incorporated by reference hereinin its entirety, the inventors have shown how multiple SBGs can bestacked together in the same waveguide and activated in rapid successionto time-sequentially tile a high-resolution, ultra-wide-field of view.Moreover, each subfield of view has the full digital resolution of theassociated imager, allowing the formation of images that approach oreven exceed the visual acuity limit of the human eye. While the tilingdisclosed in this earlier filing overcomes the twin deficiencies ofstandard guided-wave architectures: limited field of view and limitedpixel resolution, it has limitations when it is necessary to tilevertically and horizontally over large fields of view. For monochromedisplays with modest FOV and expansion in only one direction, tiling canbe accomplished by simply stacking the grating planes. However, when thefield of view is expanded in both directions and color is added, thenumber of layers needed with this approach quickly becomes impractical.Each subfield of view is limited by the diffraction efficiency andangular bandwidth of the SBG. SBG grating devices typically have angularbandwidths in air of approximately ±5° (subject to material properties,index modulation beam geometry and thickness). The inventors have foundthat larger angles can be achieved in practice by using thinner SBGs,typically smaller than 3 microns. The increased bandwidth resulting fromthinner SBGs will result in lower peak diffraction efficiency. Thereforeit is usually necessary to increase the refractive index modulation. Oneway to avoid the need for separate RGB SBGs is to use multiplexed SBGs,in which the illumination is provided from opposite ends of the lightguide as R and B/G illumination, compromising the color gamut some what.However, multiplexed gratings raise issues of fabrication complexity andcross talk.

An elegant solution to the tiling problem disclosed in United StatesProvisional Patent with a filing date of 25 Apr. 2012 by the presentinventors entitled WIDE ANGLE COLOR HEAD MOUNTED DISPLAY, is to compressthe stack by interlacing or tessellating the SBGs, as opposed to simplystacking the gratings. The display disclosed in Application No.61/687,436 comprises two elements: firstly, a multilayer waveguidedevice comprises layers of tessellated SBG arrays referred to as theDigiLens and, secondly, an optical system for providing input image datafrom one or more microdisplays referred to as an Input Image Node (IIN)which, in addition to the microdisplays, contains laser illuminationmodules, collimation and relay optics waveguide links and gratingdevices. The same terminology will be retained for the purposes ofdescribing the present invention. In very basic terms the DigiLensprovides the eyepiece while the IIN provides a compact image generationmodule that will typically be located above or to the side of theDigiLens according to the ergonomic constraints of the application. InApplication No. 61/687,436, all SBG elements sharing a givenprescription are activated simultaneously such that they diffractcollimated wave guided image light into a predetermined FOV tile. Thenumber of images that can be tiled is only limited by the input displayrefresh rate. The SBG elements would typically be a few millimeters insize. While this approach achieves significant economy in terms oflayers, it suffers from the problems of illumination ripple owing totessellated grating pattern used in the DigiLens), scatter fromelectrodes, and general optical and electrical complexity.

The motivation behind the present disclosure is to reduce the need fortessellating the DigiLens. A further problem of the prior art is thatcoupling the IIN output image into the waveguides is very inefficient,thus resulting in thick waveguides. A more efficient way of sampling theinput image field is needed overcome this problem.

SUMMARY

In view of the foregoing, the Inventors have recognized and appreciatedthe advantages of a display and more particularly to a transparentdisplay that combines Substrate Guided Optics (SGO) and Switchable BraggGratings (SBGs).

Accordingly, provided in one aspect of some embodiments is an apparatusfor displaying an image, the apparatus comprising: a first opticalsubstrate comprising at least one waveguide layer configured topropagate light in a first direction, wherein the at least one waveguidelayer of the first optical substrate comprises at least one gratinglamina configured to extract the light from the first substrate alongthe first direction; and a second optical substrate comprising at leastone waveguide layer configured to propagate the light it in a seconddirection, wherein the at least one waveguide layer of the secondoptical substrate comprises at least one grating lamina configured toextract light from the second substrate along the second direction. Theat least one grating lamina of at least one of the first and secondoptical substrates may comprise an SBG in a passive mode.

In one embodiment, the at least one waveguide of at least one of thefirst and second optical substrates comprises a plurality of gratinglaminas, at least two of the plurality having the same surface gratingfrequency.

In one embodiment, the at least one grating lamina of at least one ofthe first and second optical substrates comprises non-switching Bragggrating recorded in a HPDLC material in at least one of forward andreverse modes. While the grating lamina may be an SBG in some instances,it need not be. Other types of suitable materials may also be used.

In one embodiment, the first and second optical substrates comprise anSBG in a passive mode.

In one embodiment, at least one of the first and second opticalsubstrates comprises a plurality of waveguide layers, and each of thepluralities of waveguide layers is configured to propagate at least oneof red, green, blue, blue/green mixed light, and one of a multiplicityof sub Field of Views (FOVs). In one instance, at least one of the firstand second optical substrates comprises a plurality of waveguide layers,and when the plurality comprises three waveguide layers, the threewaveguide layers are configured to propagate red, green, and blue light.Alternatively, when the plurality comprises two waveguide layers, thethree waveguide layers are configured to propagate red light and mixedblue and green light.

In one embodiment, the at least one waveguide layer of the at least oneof the first and second optical substrates comprises holograms withsuperimposed different color prescriptions.

In one embodiment, the at least one waveguide layer in at least one ofthe first and second optical substrates is lossy.

In one embodiment, the at least one grating lamina of at least one ofthe first and second optical substrates has a thickness that is lessthan about 3 microns. For example, the thickness may be less than about2.5 microns, 2 microns, 1.5 microns, 1.2 microns, 1 micron, 0.5 micron,or even smaller.

In one embodiment, the at least one grating lamina of at least one ofthe first and second optical substrates has a varying thickness alongthe respective direction of light propagation.

In one embodiment, the apparatus described herein is a part of a device,wherein the device is a part of at least one of HMD, HUD, and HDD.

Provided in another aspect of some embodiments is an apparatus fordisplaying an image comprising: an input image node for providing imagemodulated light; a first optical substrate comprising at least onewaveguide layer configured to propagate the modulated light in a firstdirection, wherein the at least one waveguide layer of the first opticalsubstrate comprises at least one grating lamina configured to extractthe modulated light from the first substrate along the first direction;a second optical substrate comprising at least one waveguide layerconfigured to propagate the modulated light in a second direction,wherein the at least one waveguide layer of the second optical substratecomprises at least one grating lamina configured to extract themodulated light from the second substrate along the second direction.The at least one grating lamina of the first optical substrate may beconfigured to couple the modulated light into the first substrate. Theat least one grating lamina of the second optical substrate may beconfigured to couple the modulated light extracted from the firstsubstrate into the second substrate. The at least one grating lamina ofat least one of the first and second optical substrates may have ak-vector that varies along the respective direction of lightpropagation.

In one embodiment, the input image node comprises at least one ofmicrodisplay, laser, and collimating optics. A microdisplay may be anytype of microdisplay commonly used, including, for example, an emissivemicrodisplay. An emissive microdisplay may be an OLED, a QPI, and thelike.

In one embodiment, the at least one grating lamina of at least one ofthe first and second optical substrates has a varying thickness. Forexample, the thickness may increase in a direction that is at least oneof (i) parallel to a direction of the light propagation and (ii)orthogonal to the light propagation. Alternatively, the thickness mayincrease and then decrease (or vice versa) along the aforedescribeddirection. The geometry is not limited.

In one embodiment, the at least one grating lamina of at least one ofthe first and second optical substrates comprises an SBG that is in aswitching mode or in a passive mode.

In one embodiment, the at least one grating lamina in at least one ofthe first and second substrates comprises multiplex gratings of at leasttwo different monochromatic prescriptions.

In one embodiment, the apparatus comprises multiple grating laminashaving the same surface grating frequency but different k-vectors,wherein the multiple grating laminas are configured to divide the inputimage field of view into multiple angular intervals.

In one embodiment, at least one of the first and second opticalsubstrates is curved in at least one orthogonal plane.

In one embodiment, the light extracted from the first and second opticalsubstrates provides uniform illumination in any field of view direction.

Provided in another aspect of some embodiments is a method of displayingan image, the method comprising: coupling a modulated light from aninput image into a first optical substrate; extracting the light fromthe first substrate; and coupling the extracted light from the firstsubstrate into the second substrate. The first optical substrate maycomprise at least one waveguide layer configured to propagate light in afirst direction, wherein the at least one waveguide layer of the firstoptical substrate comprises at least one grating lamina configured toextract light from the first substrate along the first direction. Thesecond optical substrate may comprise at least one waveguide layerconfigured to propagate light in a second direction, wherein the atleast one waveguide layer of the second optical substrate comprises atleast one grating lamina configured to extract light from the secondsubstrate along the second direction. The at least one grating lamina ofat least one of the first and second optical substrates may comprise anSBG in a passive mode.

In one embodiment, the method further comprises sampling the input imageinto a plurality of angular intervals, each of the plurality of angularintervals having an effective exit pupil that is a fraction of the sizeof the full pupil. In one stance, this surprisingly provides anadvantage that the thickness of the first waveguide can be much smallerin comparison to pre-existing devices. Accordingly, the size andplacement of the input gratings may be advantageously affected.

In one embodiment, the method further comprising improving thedisplaying of the image by modifying at least one of the following ofthe at least one grating lamina of at least one of the first and secondoptical substrates: grating thickness, refractive index modulation,k-vector roll profile, surface grating period, and hologram-substrateindex difference.

Provided in another embodiment is an apparatus for displaying an imagecomprising: an input image node for providing image modulated light;first and second optical waveguiding substrates; a first optical meansfor coupling image modulated light into said first substrate; and asecond optical means for coupling light extracted from the firstsubstrate into the second substrate. The first optical substratecomprises at least one waveguiding layer that propagates light in afirst direction. Each waveguiding layer contains at least one gratinglamina operative to extract light from the first substrate, the lightextraction taking place along the first direction. The second opticalsubstrate comprises at least one waveguiding layer. Each waveguidinglayer propagates light in a second direction. Each waveguiding layercontains at least one grating lamina operative to extract light fordisplay from the second substrate, the light extraction taking placealong the second direction. In one embodiment the first opticalsubstrate selectively samples portions of the image modulated light,each portion being characterized by either angular field or spatialfield.

It should be appreciated that all combinations of the foregoing conceptsand additional concepts discussed in greater detail below (provided suchconcepts are not mutually inconsistent) are contemplated as being partof the inventive subject matter disclosed herein. In particular, allcombinations of claimed subject matter appearing at the end of thisdisclosure are contemplated as being part of the inventive subjectmatter disclosed herein. It should also be appreciated that terminologyexplicitly employed herein that also may appear in any disclosureincorporated by reference should be accorded a meaning most consistentwith the particular concepts disclosed herein.

BRIEF DESCRIPTION OF THE DRAWINGS

The skilled artisan will understand that the drawings primarily are forillustrative purposes and are not intended to limit the scope of theinventive subject matter described herein. The drawings are notnecessarily to scale; in some instances, various aspects of theinventive subject matter disclosed herein may be shown exaggerated orenlarged in the drawings to facilitate an understanding of differentfeatures. In the drawings, like reference characters generally refer tolike features (e.g., functionally similar and/or structurally similarelements).

FIG. 1A is a schematic illustration of the optical geometry of a Bragggrating in the tangential plane.

FIG. 1B is a schematic illustration of the optical geometry of a Bragggrating in the sagittal plane.

FIG. 2A is a schematic side elevation view showing a first operationalstate in one embodiment.

FIG. 2B is a schematic side elevation view showing a second operationalstate in one embodiment.

FIG. 2C is a schematic front elevation view of one embodiment.

FIG. 3 is a schematic three dimensional view of the components of oneembodiment.

FIG. 4 is a schematic side elevation view of the components of oneembodiment.

FIG. 5 is a flow chart representing the formation of an image in oneembodiment.

FIG. 6 is a schematic side elevation view of one embodiment.

FIG. 7 is a schematic side elevation view of one embodiment.

FIG. 8 is a schematic side elevation view of one embodiment.

FIG. 9 is a chart showing the diffraction efficiency characteristics ofone embodiment.

FIG. 10 is a schematic cross sectional view of a horizontal beamexpander in one embodiment.

FIG. 11 is a table provide diffraction efficiency characteristic of SBGsused in one embodiment.

FIG. 12A is a schematic three dimensional view showing a firstoperational state of one embodiment.

FIG. 12B is a schematic three dimensional view showing a secondoperational state of one embodiment.

FIG. 13A is a schematic three dimensional view showing a thirdoperational state of one embodiment.

FIG. 13B is a schematic three dimensional view showing a fourthoperational state of one embodiment.

FIG. 14A is an artist's impression of a first aspect of a HMD implementof the invention.

FIG. 14B is an artist's impression of a second aspect of a HMD implementof the invention.

FIG. 14C is an artist's impression of a third aspect of a HMD implementof the invention.

FIG. 15 is a schematic cross section view of a wedged profile waveguidegrating used in one embodiment.

FIG. 16 is a schematic side elevation view of one embodiment.

FIG. 17 is chart shown diffraction efficiency versus anglecharacteristics of one embodiment.

FIG. 18 is a schematic side elevation view of one embodiment.

FIG. 19A is a schematic side elevation view showing a first operationalstate of one embodiment.

FIG. 19B is a schematic side elevation view showing a second operationalstate of one embodiment.

FIG. 20A is a schematic side elevation view showing a first operationalstate of an input image node in one embodiment.

FIG. 208 is a schematic side elevation view showing a second operationalstate of an input image node in one embodiment.

FIG. 21 is a schematic plan view of a HBE containing slanted gratingstripes.

FIG. 22 is a schematic three dimensional view of one embodiment using aHBE containing slanted grating stripes.

FIG. 23A is a schematic side elevation view showing a first operationalstate of a HUD provided by one embodiment.

FIG. 23B is a schematic side elevation view showing a second operationalstate of a HUD provided by one embodiment.

FIG. 24 is a schematic side elevation view of a HUD provided by oneembodiment.

FIG. 25 is a schematic side elevation view of a HUD provided by oneembodiment.

FIG. 26 is a schematic illustration of a prior art collimated imagedisplay.

FIG. 27 is a schematic side elevation view of a HUD according to theprinciples of the invention.

FIG. 28 is a schematic side elevation view of a holographic mirror.

FIG. 29 is a schematic side elevation view of a HUD provided by oneembodiment.

FIG. 30 is a schematic side elevation view of a HUD provided by oneembodiment.

FIG. 31 is a schematic side elevation view of a HUD provided by oneembodiment.

FIG. 32A is a three dimension view of a color display provided by oneembodiment.

FIG. 32B is a chart showing the spectral characteristics of a lightsource used in a color display provided by one embodiment.

FIG. 32C is a table showing the spectral characteristics of a lightsource used in a color display provided by one embodiment.

FIG. 33 is a cross sectional view of a HUD integrated in a windscreen inone embodiments of the invention.

FIG. 34 is a cross sectional view of a HUD integrated in a windscreen inone embodiments of the invention.

FIG. 35 is a three dimensional view of a display provided by oneembodiment.

FIG. 36 is a three dimensional view of a display provided by oneembodiment.

FIG. 37 is a schematic side elevation view of a color display providedby one embodiment.

FIG. 38 is a schematic three dimensional view of a color displayprovided by one embodiment.

FIG. 39A is a schematic side elevation view of one embodiment.

FIG. 39B is a schematic front elevation view of one embodiment.

FIG. 40 is a cross sectional view of a HUD integrated in a windscreen inone embodiment.

FIG. 41 is a flow chart illustrating image formation in a HUD in oneembodiment.

FIG. 42A is a chart show a first diffraction efficiency versus anglecharacteristic of a multiplexed DigiLens in one embodiment.

FIG. 42B is a chart show a second diffraction efficiency versus anglecharacteristic of a multiplexed DigiLens in one embodiment.

FIG. 43 is a schematic three dimensional view of a color multiplexeddisplay in one embodiment.

FIG. 44 is a schematic side elevation view of a DigiLens used in a colormultiplexed display in one embodiment.

FIG. 45 is a three dimensional illustration of an embodiment of adisplay in one embodiment in which there are provided three HBEwaveguides and three DigiLens waveguides.

FIG. 46 is a schematic side elevation view showing the formation of aprojected stop by the IIN.

FIG. 47 is a schematic plan view illustrating the coupling of light fromthe IIN into the HBE waveguide.

FIG. 48 shows a detail of the HBE waveguide of FIG. 47 . The image inputend illustrating the interaction of the beam with the gratings.

FIG. 49 is a schematic cross-sectional view of a four layer HBE in oneembodiment.

FIG. 50 is a table showing the gratings used in the embodiment of FIG.49 .

FIG. 51 is a chart showing overlapping DE versus angle profiles in theembodiment of FIG. 49 .

FIG. 52 is a three dimensional view of a wedge SBG grating in oneembodiment.

FIG. 53A is a schematic illustration of a first operational state of theHBE in one embodiment.

FIG. 53B is a schematic illustration of a second operational state ofthe HBE in one embodiment.

FIG. 53C is a schematic illustration of a third operational state of theHBE in one embodiment.

FIGS. 54A-54B illustrate projection schemes in one embodiment that donot result in a projected pupil of the type discussed earlier in thedescription; the pupil may be formed inside the projection lens (FIG.54A) or before the projection lens using the aperture 562 (FIG. 54B).

FIG. 55 shows a schematic illustration of the use of rolled k-vectorgratings to maximize the peak DE of in-couple light in one embodiment.

FIG. 56 shows a schematic illustration showing the propagation of atypical ray through a waveguide section 580 containing rolled k-vectorgratings in one embodiment.

FIG. 57 shows a plan view showing the HBE 590 and the VBE 591 in oneembodiment.

FIG. 58 shows a schematic side elevation view of the HBE and the VBE inone embodiment.

FIG. 59 shows an unfolded view of the HBE showing the beam propagationinside the waveguide in one embodiment.

FIG. 60 illustrates an apparatus for fabricating the HBE using a contactcopying process in one embodiment.

FIGS. 61A-61C, respectively, show a cross section of the Z=L end of theHBE 590 with the wider end of a cone shaped lens overlaid, a plan viewof the lens, and the Z=0 end of the HBE with the narrower end of thelens overlaid in one embodiment.

FIGS. 62A-62C illustrate the generation of the conic section from a coneof tip 620 and base 621; the cone is shown in side view in FIG. 62A andin-front view in FIG. 62B; a view of the cut out optics projected fromthe base along the cut line is shown in FIG. 62C.

FIG. 63 shows the basic architecture of a waveguide in one embodiment.

FIG. 64 is an illustration of a waveguide in which the input gratings635A-635C are stacked. Each grating has a unique k-vector 636A-636C inone embodiment.

FIG. 65 shows that the input gratings are disposed adjacent to eachother along the waveguide propagation direction in one embodiment.

FIG. 66 illustrates that the principles illustrated in FIGS. 64-65 mayalso be applied in the output grating in one embodiment.

FIG. 67 shows a flowchart describing a process of one embodiment.

FIG. 68 shows a ray trace of a monochromatic version of the design inone embodiment.

FIG. 69 shows the approximate dimensions of the IIN of FIG. 63 in oneembodiment.

FIG. 70 provides unfolded views of the optical layout of FIG. 64 in oneembodiment.

FIGS. 71A and 71B illustrate the formation of a projected stop insidethe HBE in one embodiment using a simplified thin lens representation ofthe microdisplay projection optics.

FIG. 72 illustrates one embodiment, in which the HBE that comprises acoupling grating at its input end and alternating SBG stripes of twodifferent prescription, is inclined at 45 degrees.

FIG. 73 illustrates beam propagation from the IIN through a single layerof the DigiLens showing the four changes in direction that occur alongthe path to the exit pupil in one embodiment.

FIG. 74 shows plan view of the near eye geometry of the proposed helmetmounted display in one embodiment.

FIG. 75 shows side view of the near eye geometry of the proposed helmetmounted display in one embodiment.

FIG. 76 shows front view of the near eye geometry of the proposed helmetmounted display in one embodiment.

FIG. 77 shows the relationship of the DigiLens® aperture to the FOV, eyerelief and eye box in one embodiment.

FIG. 78 shows partial binocular overlap in one embodiment.

FIG. 79 shows partial binocular overlap in another embodiment.

FIG. 80 shows a transparent waveguide display in one embodiment.

DETAILED DESCRIPTION

Following below are more detailed descriptions of various conceptsrelated to, and embodiments of, inventive a transparent display. Itshould be appreciated that various concepts introduced above anddiscussed in greater detail below may be implemented in any of numerousways, as the disclosed concepts are not limited to any particular mannerof implementation. Examples of specific implementations and applicationsare provided primarily for illustrative purposes.

The invention will now be further described by way of example only withreference to the accompanying drawings. It will apparent to thoseskilled in the art that the present invention may be practiced with someor all of the present invention as disclosed in the followingdescription. For the purposes of explaining the invention well-knownfeatures of optical technology known to those skilled in the art ofoptical design and visual displays have been omitted or simplified inorder not to obscure the basic principles of the invention. Unlessotherwise stated the term “on-axis” in relation to a ray or a beamdirection refers to propagation parallel to an axis normal to thesurfaces of the optical components described in relation to theinvention. In the following description the terms light, ray, beam anddirection may be used interchangeably and in association with each otherto indicate the direction of propagation of light energy alongrectilinear trajectories. Parts of the following description will bepresented using terminology commonly employed by those skilled in theart of optical design. It should also be noted that in the followingdescription of the invention repeated usage of the phrase “in oneembodiment” does not necessarily refer to the same embodiment.

The present invention is made possible by two fundamental properties ofSBGs that have not been exploited to date firstly the relatively wideangular bandwidth of Bragg gratings in the plane orthogonal to the planeof diffraction and secondly the wide angular bandwidths resulting frommaking SBGs very thin. As a result, the constraints of limiting the sizeof FOV tiles to around 10°×10° does not apply in this instance, therebyleading to the tessellation approach discussed above. Fewer bigger tilesmay now be used as a result. As is shown in the following descriptionthe needed FOV may be divided into two tiles with one DigiLens for each.Other numbers of tiles may also be possible. With respect to the opticaldesign this new approach may minimize, if not eliminate entirely, theproblem of illumination ripple. By making the DigiLens passive theproblems of scatter firm electrodes and the not insignificant problemsof wiring up large matrices of tessellation elements may be avoided. Apassive SBG is no different from a switching SDBG in terms of its HPDLCformulation and recording process. The only difference is that noelectrodes are needed. The diffracting properties of an SBG are normallyspecified in the tangential plane. In a grating design to diffract lightin a plane, the tangential plane is the plane containing the incidentand diffracted ray vectors and the grating vector. Following geometricaloptical theory the plane orthogonal to the tangential plane is referredto as the sagittal plane. FIG. 1 illustrates the basic geometricaloptics of a transmission SBG 90 containing slanted fringes such as 91with grating vectors K aligned normal to the fringes. In Bragg gratingsthe a multiplicity of input and output rays will satisfy the Braggcondition provided the angles between the incident rays and the k-vectordiffracted rays and the K-vector satisfy the Bragg equation. (Note thatin practice, according to the Kogelnik theory of Bragg gratings,reasonably high diffraction will be obtain for off-Bragg angles having asmall angular or wavelength deviation from the on-Bragg ray directions).In FIG. 1 these off-Bragg rays are illustrated by the ray cones 902,903surrounding the on-Bragg (lying in the in-plane of the drawing) rays900,901. As shown in FIG. 1A the locus of the on-Bragg ray-fringeintercepts is the circle 904. As shown in FIG. 1B rays 905,906 will alsobe on-Bragg. From consideration of the geometry of FIG. 1A it should beapartment that the Bragg diffraction angular bandwidth in the tangentialplane is limited by the projections of the cones 902,903 onto thetangential plane. However, turning to FIG. 1B it should be apparent theeffective angular bandwidth (“ABW”) in the sagittal plane is much largeis it is provided by the projection of cone 904 into the sagittal plane.In practice the sagittal bandwidth is mainly limited by the TIR angleconstraints set by the waveguide. As a consequence of the large sagittalplane (i.e. horizontal plane for our purposes) angular bandwidth ofBragg gratings (typically around 4×the tangential bandwidth) currenthorizontal POV targets may be achieved for most display applications. Inpractice the bandwidth is only limited only by TIR angle range that canbe sustained in the waveguide.

The inventors have already demonstrated that thin SBG gratings providevery wide angular bandwidths. An experimental SBG waveguide made using alow index modulation SBG RMLCM formulation has been shown to have a FWHMbandwidth of 21° with a 1 micron thick SBG layer.

In the following description many references to gratings are made, whichshould generally be understood to mean a Bragg grating and desirably aSBG. In many cases the SBGs will be operated in their normal switchingmode as described above. However, in some cases SBGs will be used in apassive (e.g., completely passive) mode that is they will not beswitchable. A non switching SBG is superior to a conventional passivehologram for the reason that the LC component of the HPDLC entanglesmuch higher refractive index modulations than can be achieved inconventional holographic photopolymers. In certain embodiments of theinvention the display will use a mixture of switching and non switchingSBGs. The DigiLens output gratings will always be passive(non-switching), however. In one particular class of embodiments thedisplays will use all passive SBGs.

A transparent display according to the principles of the invention isillustrated schematically in FIG. 2 . The DigiLens®, which provides athin highly transparent eye piece (or HUD combiner) comprises twowaveguides 101,102 for projecting the upper and lower halves of thefield of view into the eye box (not shown). The waveguides each comprisenon switchable SBG layers sandwiched between transparent substrates.Each waveguide has a switchable input grating and a non switching(passive) output grating labelled as DIGI-I1, DIGI-O1 and DIGI-I2,DIGI-O2 which are also indicated by the numerals 107,109 and 108,110respectively. The waveguides are separated by a Half Wave Film (HWF)106. (Note than in other embodiments to be described below the HWF willbe disposed between the DIGI-I gratings and the DIGI-O gratings will beair (or low-index material) separated). An input image node (IIN) 103which will be discussed later contains the microdisplay, laser module,beam expansion, collimation and relay optics. Schematic side elevationviews are provided in FIGS. 2A-28 and a front elevation in FIG. 2C.FIGS. 2A-23 indicate the ray paths from the IIN through the DigiLenslayers for the two switched states of the display. In the first statethe grating DIGI-I1 is active and diffracts incident P-polarised light1000 from the IIN 103 into the TIR path 1001. The TIR light isdiffracted out of the waveguide along its light as indicated by 1002.The output grating is lossy, that is the diffraction efficiency issignificantly less than unity such that a portion of the guide lightgets diffracted out at each beam-grating interaction. The remaininglight continues to undergo repeated TIR and diffraction until all of thelight has been extracted from the waveguide. Uniform illumination acrossthe output aperture is achieved by careful optimisation of diffractionefficiency (which depends on the refractive index modulation, gratingthickness and other parameters). In general low diffraction efficiencyis needed at the end of the waveguide nearest the IIN and the highestefficiency at the extreme end. Note that due to lossy extraction morepeak energy (at 0°) is coupled into the DigiLens than at higher angles.Thus wider angle light is available for extraction at the end of a lossygrating. While the phrase “lossy grating” is employed in someembodiments, the phrase encompasses “lossy waveguide. Not to be bound byany theory, but this is because the “lossy” may be due to a combinationnot the grating efficiency and waveguiding action that may result in theuniform loss along the wavefuid.

This helps to homogenize peak and edge angular variations, particularlyat the thicker end of the waveguide where the DE curve narrows. Thediffracted light 1002 has its polarisation rotated through 90 degrees(becoming S-polarised) by the HWF and therefore passes the secondwaveguide 102 without deviation since SBGs have relative low DE forS-polarised light. Note that one DigiLens® layer emits S-polarized lightwhile the other emits P-polarised light. However, each SBG layer isP-diffracting.

The Horizontal Beam Expander (HBE) indicated by the labels HBE1, HBE2(also referenced by the numerals (104,105) is a multilayer SBG waveguideusing lossy high ABW gratings to expand the image light across a largepupil. In the above described embodiment the HBE runs along the top edgeof the DigiLens. The HBE will be discussed in more detail later. Notethat air gap between the front and rear DigiLens® elements. This may bereplaced by a suitable low (near unity) index material. Since the outputimage light is a mixture of P and S polarized light it may be necessaryto mount a quarter wave film on the output surface of the DigiLens forcompatibility with Polaroid type eye ware which would otherwise resultin the loss of half of the field of view.

Although it is referred to an HBE (and a VBE in an earlier filing) theterms horizontal and vertical in this context only have significance forthe purposes of illustrating the invention. In practice the inventionallows many different configurations of the comments and severaldifferent ways of implement the bean steering the beam expansion may bevertical or horizontal. With regard to the term waveguide it should benoted that these may actually comprise multiple isolated waveguidesstacked in layers. Finally with regard to grating components it shouldbe understood that each of the three grating components may containmultiple gratings stack in layers, disposed adjacently in a single layeror holographically multiplexed in a single layer. The basic buildingblock of the displays discloses is a waveguide containing a grating,normally a Bragg grating. As will be seen the function can in certainembodiments be accomplished with as few as one waveguide layer. Howeverthe number of waveguide layers will depend on the size of field of viewand the color needed. The grating may be switchable (SBG) or it may bepassive, that is, non switchable. Although in principle, any type ofBragg grating may be used to provide a passive grating. There is astrong advantage in using an SBG with no electrodes. SBG material hasthe advantage that the mixture of LC and polymer affords higherrefractive index modulation than that of conventional holographicpolymer materials. In the preferred embodiment of the invention n theoutput waveguide component uses only non tessellated passive gratings.This minimizes the potential problems of scatter from electrodes andillumination non uniformities. T term grating is employed to refer to aBragg grating unless otherwise specified. Passive grating means agrating that is not electrically switched.

The display is shown in more detail in FIGS. 3-4 . As a further aid tounderstanding how a collimated display (e.g., HMD) works, the initialfocus is on the monochrome version of the design. Architecturally, themonochrome and color implementations of the HWD are very similar. Aswill be seen, an important difference is that the monochromearchitecture can be achieved with fewer waveguiding layers and thepossibility of using some passive grating components in the IIN and HBE,while a color implementation needs most components of the IIN and HBE tobe switchable owing to the greater difficulty of managing the angularcontent of red, green and blue optical channels simultaneously. In bothcases the DigiLens® remains a passive component.

While the present invention has many applications in the field oftransparent visual displays it is first considered one particularapplication namely a Helmet Mounted Display for Augmented Reality (AR)application. The objective in this case is to meet the 52° H×30° Vmonocular field of view specification while achieving all of ouroriginal goals of high transparency, high resolution, ultra compact(thin) form factor, light weight and generous exit pupil. The targetspecifications are summarized in Table 1.

TABLE 1 Target HMD Specifications. Helmet Mounted Display SpecificationColor Monochrome - Extendable to Full Color Total Field of View (FOV)85° H × 30° V Monocular Field of View 52° H × 30° V Binocular Overlap20° Eye Box 25 mm. × 25 mm. Eye Relief 25 mm. Resolution 1080p FormFactor Ultra compact (thin) DigiLens Active Area 49.4 mm. × 33.4 mm.Transparency >95%

The important components of the display are illustrated in the schematicthree dimensional drawing of FIG. 3 and the side elevation view of FIG.4 . The display splits the FOV into upper and lower FOV tiles (referredto by the numerals 1, 2 in the drawing labelling) Note that thewaveguide substrates of the DigiLens and HBE components have not beenshown in order to simplify the explanation. The display comprises aDigiLens® comprised of two waveguide layers sandwiching a HWF is splitinto input and output components DIGI-I and DIGI-O. Note that widesagittal angular bandwidth of SBGs removes the need to tilehorizontally. Two Horizontal Beam Expanders HBE each comprising inputgratings HBE-I and output gratings HBE-O are provided. The expandedoutput light from HBE-O1 enters the first DigiLens waveguide viaDIGI-I11 and similarly for the second waveguide. Note that the abovecomponents are also referenced by numerals 130-145 in FIGS. 3-4 . TwoIIN are provided: one for the upper FOV and one for the lower FOV. Thedisplay panel in each IIN is a 1080p 5 mm×3 mm LCoS devices. One lasermodule may be used to illuminate both display panels. However, theinvention does not place any restriction on the number of microdisplaysto be used. A single microdisplay with a fast enough refresh rate andhigh enough resolution is likely to be sufficient for all but the mostdemanding display applications.

The DIGI-I is the most challenging grating in the system since it needshigh input coupling efficiency at the projected pupil output point fromthe HBE-O, across the full angular range. The DIGI-I gratings switch,sampling the 52° horizontal×30° vertical field output by the HBE-O intothe two DigiLens waveguides. It is desirable that this grating needs ahigh angular bandwidth and high DE. The DIGI-I comprises 2 SBGs eachoperating over 8.5° angular bandwidths overlapping to provide at least15°. DIGI-I uses two 3 micron SBGs of DE approximately 87% with angularbandwidth of 8.5°-9.0° in air. The vertical field from −15° to 0° isswitched by DIGI-I1 and the vertical field from 0° to +15° into DIGI-2.Hence DIGI-I1 provides 52° horizontal×−15° vertical and DIGI-I2 provides52° horizontal×+15° vertical. All gratings in the DIGI-O are passive,and therefore can be thin gratings. One of each pair is for red and theother for blue/green. DIGI-O1 the rear grating providing the lower 15°and the front grating DIGI-O2 providing the upper 15° giving a total 52°horizontal×30° vertical. As shown in FIG. 4 the DigiLens® is tilted at arake angle of ˜8-10°. This is found from ray-tracing analysis to givebetter DE than simply projecting image light normally into theDigiLens®.

A flow chart representing the interaction between the IIN, HBE andDigiLens in the image formation process is provided in FIG. 5 . Sincediffractive optical elements are dispersive it usually desirable wheremore than one grating is combined to configured them in a complementaryfashion such that the dispersions introduced by the gratings cancel.Complementarily is normally achieved by designing the gratings to havethe same grating pitch (that is, the spatial frequencies of theintersections of the Bragg gratings with the substrates are identical).It should be noted that HBE-I2 and HBE-O2 need to be complementary inthe embodiment described above. However, HBE-I1 and HBE-I2 do not needto be complementary.

In one embodiment shown in FIG. 6 the HWF between the DigiLenswaveguides (overlapping the DIGI-O gratings) is removed and a HWD isinserted in the spaced between the DigiLens DIGI-I input gratings. Theair gap left by the HWP may be filled with a low index nanoporousmaterial. Quarter Wavelength Film (QWF) is applied to opposing faces157,158 of the front and rear waveguides with the effect that each TIRbounce results in a 90° polarization rotation allowing an approximately×4 thinner grating and no interaction between the front and rearDigiLens®. FIG. 7 shows the propagation of upper and lower FOV light inthe two DigiLens waveguides as represented by the ray paths 1010, 1012,1014 in the first waveguide and 1011, 1013, 1015. The components shownin FIGS. 6-7 are also referenced by the numerals 150-159. FIG. 8 is aview of one of the DigiLens waveguides 160 illustrating the function ofthe QWF layer 162 in more detail. Input light from the HBE 1020 isdeflected into the TIR path 1021 by the DIGI-I grating 161. Rays such as1022 incident on the QWF coating have their polarization converted fromP to circularly polarized light of a first sense. On reflection thepolarization remains circular but in an opposing sense such that afterpassing through the QWF the second time the light emerges as S-polarized1023. The S light is not diffracted by the SBG and therefore continuesto undergo TIR. On the next reflection at the QWF film the light isconverted to P polarized light 1024, which is off-Bragg with respect toDIGI-I and so does nor get diffracted back towards the HBE. The TIR ofthe beam then proceeds onto the DIGI-O grating where it is progressivelyextracted from the waveguide as described above.

In one embodiment the two stacked DIGI-I gratings may be provided ineach DigiLens waveguide to increase the angular bandwidth. FIG. 9 is achart showing the effect of combining the individual DE angularbandwidths to create upper and lower FOVs of approximately 15 degrees(FWHM) in air using two gratings of angular bandwidth 8.5 degrees (FWHM)in air. In other embodiments of the invention the DigiLens couldcomprised more layers, for example 3 DIGI-I layers combined with 2DIGI-I layers in each waveguide. Note that DIGI-I and DIGI-O gratings dono need to be co-planar. However in fabrication terms it is advantageousto limit the number of grating, substrate, electrode layers and lowindex material layers to minimize material costs and process steps.

FIG. 10 shows the HBEs in cross section in relation to the DigiLens 170.There are two HBE waveguides 171, 172 each comprising 3 stacked gratings(HBE-I1A-C and HBE-I2A-C) and two lossy output gratings (HBE-O1A-B andHBE-O2A-B). The HBEs are coupled to the DigiLens 170. Two IINs (IIN1 andIIN2) are provided. The paths of light from the IIN to the DigiLens areindicated by rays 1030,1031,1034 and 1032,1033,1035. Note the componentsare also reference by numerals 170-183. Each LCoS provides a FOV of26°H×30° V. Each HBE-I contains three gratings that operate onP-polarized light in 8.5° ABW steps to provide the 26° half horizontalfield. The 30° field will couple in its entirety owing to the muchincreased ABW in the sagittal plane. The HBE-I SBGs are thick gratingsallowing high DE but narrow ABW. There are two HBE-I implementationoptions to be considered: firstly, 26° H sampled by 2 HBE-I gratings,which gives lower DE, higher duty cycle SBGs; or, secondly, 26° Hsampled by 3 HBE-I gratings, giving 3×8.5° angular bandwidth. This giveshigher DE, lower duty cycle SBGs. The gratings are typically ofthickness 1-2 micron and lossy such that light is extracted with uniformefficiency along the length of the grating. Lossy gratings have largeABW and low DE. The gratings of HBE-I1 and HBE-I2 do not need to becomplementary (i.e. chromatic dispersion correcting). Gratings of HBE-I1and HBE-O1 (as well as HBE-I2 and HBE-O2) need to be complementary. In apassive HBE-I a single unswitched grating needs to be thin to achievehigher angular bandwidth. With current materials roughly 30% peak DEwith current materials, and 60% is within the range of expected materialimprovements may be achieved. In the case of a switching HBE-I a thickergrating of reduced angular bandwidth may be afforded. With currentmaterial refractive index modulations, the angular bandwidthapproximately halves and DE doubles as grating thickness is increasedfrom 1.4 to 2.0 micron. Typical DE and ABW characteristics of thin andthick SBGs are summarized in the table in FIG. 11 .

FIGS. 12-13 provide a walkthrough of the process of projecting imagelight from one (left eye) IIN into the eyebox providing ±5° vertical and0-26° horizontal field of view. The components are identical to the onesshown in FIGS. 3-4 . FIG. 12A illustrates the path 1040 from the LCoSpanel to the output of the IIN. FIG. 12B shows the light path throughthe HBE indicating the TIR path 1042 within one of the waveguides andlight extraction 1043 along its length. FIG. 13A shows the coupling oflight 1044 extracted from the HBE into the DigiLens (vie DIGI-I).Finally FIG. 13B shows the downward propagation of light 1045 in theDigiLens with the output put image light 1046 providing the lower halfof the FOV.

FIGS. 14A-14C shows three 3D views of the above invention implemented ina HMD 190. Threes difference perspectives 191-193 are shown. The displaymodule includes a horizontal hinge 194. In the deployed position, theuser will have full panoramic see-through with high transparency. In thestowed position, the user will be free to use range finders,night-vision systems, or other such equipment. As indicated in thedrawings spectacle wearers are accommodated and the design will can alsoaccommodate the use of Polaroid eyewear. In an alternative embodiment asimple display retracting mechanism allows the display to be hidden in acompact module under the brow of the helmet when not in use. In yetanother embodiment of the invention the display when not in use isretracted vertically into the helmet. The currently preferredimplementation uses a custom helmet is linked to a belt pack via anumbilical fiber-optic communications link and power supply connection.In training applications the belt pack would be wirelessly linked to thetraining center.

An important feature of the optical design is that the gratings used inone or both of the HBE and DigiLens will have a tailored DE profileachieved by varying the thickness of the gratings along the propagationdirection as shown in FIG. 15 . The wedge grating 203 is provided byincluding a small wedge in of the grating substrates 201. The secondsubstrate 202 may be rectangular. Other ways of achieving wedge gratingswill be apparent to those skilled in the art of optics. Where twostacked gratings are used the DE profiles of the two gratings would runin opposing directions. Desirably, the grating thickness may vary from1.0-1.2 micron up to 2.8-3.0 micron, the lower thickness producing thelargest bandwidth 1052 and lowest DE for a given output direction 1053.The higher thickness produces the lowest bandwidth 1050 and higher DEfor a given output direction 1051. Note that the wedge angles are tinyand will have minimal impact on illumination uniformity and imagequality.

A refractive index of approximately 1.585 is needed to support waveguideTIR angles typically not greater than 70° with respect to the TIRsurface. In general it is desirable to limit the use of higher angleswithin the waveguide to avoid low numbers of interactions of the raybundle with outcoupling grating which creates gaps in the waveguide.Higher angles (approximately 85°) can lead to image fold over wheregratings are designed to provide very high angular bandwidth.Polycarbonates will enable a TIR angles up to approximately 72°.

While a monochrome display can be achieved with mostly passive gratingcomponents, in the case of a full color display HBE-I and HBE-O and theDigiLens input gratings DIGI-I are active with the only passive gratingbeing the output grating DIGI-O. A further difference between monochromeand color HMDs is that in the latter separate waveguides are used in theHBE and to cover red and blue/green wavelengths. FIG. 16 illustrates aDigiLens used in an embodiment of the invention for color displayscomprising two DigiLens doublet waveguides. Each doublet waveguide issimilar to the embodiments of FIGS. 6-7 . However, in each DigiLenswaveguide doublet one of the waveguides operates on red light and thesecond one operates on a mixture of blue and green light. Note also thatin each doublet, the red grating is placed after the B/G grating, i.e.closer to the eye. The input and output DigiLens gratings arerepresented by DIGI-IR and DIGI-OR in the case of the red doublet andDIGI-IB/G and DIGI-OB/G in the case of the blue/green doublet. Theoutput grating portions of the waveguides sandwich a HWF. The inputgrating portions may sandwich an air gap as illustrated or preferably alow index material. QWFs are applied to the opposing face of thewaveguides in each doublet. The components are also labelled by numerals210-227. The red and blue/green waveguides are optically separated byair or a low index (near unity) material such as mesoporous silica whichare not illustrated but will be described later. A HWF converts the rearoutput from P to S. As SBG gratings are P-sensitive, this preventsre-coupling of the light with gratings in front. The rake angle (8-10°)affords higher angular bandwidth, and lower chromatic dispersionenabling shared blue/green gratings. In most cases color imaging mayneed high index substrates and special coatings for enhancing the blueTIR angular range.

The ray paths for red light are indicated by the rays 1071, 1073, 1075.The ray paths for the blue/green light are indicated by 1070, 1072,1074. As shown in the drawing, some of this light will couple into thesecond waveguide doublet, that is the light paths indicated by 1076,1078 (blue/green) and 1077, 1079 (red). The risk of light diffractedfrom the rear waveguide interacting with the light on the layer aboveand coupling back into the waveguide is avoided in our HMD design. Thered and blue/green gratings do not cross-couple due to the polarizationmanagement. Each color channel can cross couple with itself. However,this is mitigated by TIR occurring in the forward grating andreciprocity ensuring that outcoupling is in the correct outputdirection. The offset of the front and rear out-coupled beams due thestaggered path helps to homogenize the output light. The effects onthroughput of light getting re-coupled back into the DigiLens® arenegligibly small. To provide immunity from grating coupling a HWP can beintroduced at one layer in the passive waveguide stack. A half waveretarder layer converts the rear output light from P to S. The SBGgratings are P-sensitive only, and so this prevents re-coupling of thelight with gratings in front. A 10° rake angle alleviates demands ongrating prescription affording higher ABW and lower chromaticdispersion. This enables shared blue/green gratings. However, in mostembodiments of the invention red may need a separate grating.

The graph in FIG. 17 shows calculated DE versus angles for each gratingand the output DE for the layer minus the light coupled back into TIR.It is assumed that this light is not coupled back out again. Thecomposite output of the gratings including the single interactioncoupling loss is represented by circular symbols. Note that with the2*1/e offset of the peak DE profile, and accounting for re-couplingeffects of the rear grating into the front grating, that an effectivedoubling in the FWHM of a single grating is achieved. With secondaryoutput coupling of re-coupled light, the profile will approach thelossless composite grating profile (triangular symbols).

FIG. 18 shows a further embodiment of the invention for color image. TheDigiLens comprises two separated monochromatic doublet waveguides230,231 one for red (DIGI-O1A,DIGI-O1B and one for blue-green(DIGI-O2A,DIGI-O2B). The input SBGs (DIGI1A-1D, DIGI2A-2D) comprising astack of four monochromatic red or blue-green gratings indicated by A-D.In all other respects the architecture is very similar to the embodimentof FIG. 16 . HWF and QWFs are disposed as in FIG. 16 . The doublets maybe air separated or may sandwich a low index material. The componentsare also labelled by the numerals 230-246.

In an alternative embodiment of the invention similar to the one of FIG.1 shown in FIG. 19 each DigiLens waveguide comprises a single SBG layerthat supports red, green and blue TIR. Starring from the IIN the pathsare illustrated by numerals 1080,1082,1083 in the first waveguide 250and 1081,1084,1085 in the second waveguide 251. In each case the red,green and blue paths are referenced by characters R,G,B. The systemcomponents are labelled as in FIG. 1 and additionally referenced in FIG.19 by the numerals 250-258. It should be appreciated that such animplementation of the invention needs careful control of the TIR anglesto ensure that the diffracted light paths for the three colors overlapexactly. The inventors have found that additional coatings may be neededto improve the reflection at the blue end of the spectrum. In analternative embodiment of FIG. 19 the DIGI-O gratings could beimplements as a multiplexed grating.

An IIN design for use with the invention is shown in FIG. 20 . Theoptical system comprises a waveguide 260 containing overlapping SBGelements 261,262, overlapping SBG elements 263,264 a beam splitter layer265 a curved mirror 266, a prism 267 a projection lens 268 and amicrodisplay panel 269. An air gap 270 between the curved mirrorelements is provided to enable TIR of reflected light. As illustratedseparate SBG input and output gratings are provided for each imagefield. The waveguide 260 and the gratings 263,264 in particular mayprovide the input gratings of the HBE. Alternatively it waveguide 260may be used to couple light into the input grating of the HBE. It shouldbe appreciated that the IIN may be configured in many different ways tosatisfy constraints of space, cost and optical efficiency. In theembodiment illustrated half the image from the microdisplay is imagedinto the HBE sequentially. Hence in FIG. 20A the gratings 261, 264 arein their active state and the others are inactive. Light from the imageportion 1090 is projected into the path 1091 by the projection lens. Itis then reflected by the prism into the ray path 1092 reflected at thecurved mirror 266 into the path 1093 diffractive by grating 261 into thepath 1094 undergoes TIR into the path 1095 and is diffracted out of thewaveguide by the grating 264. FIG. 208 illustrate the light paths fromthe second image field 1097. Now the gratings 261,264 are switched totheir inactive states and gratings 262,263 are switched to their activestates. The path from the microdisplays is indicated by 1098-1104. Insome embodiments of the invention the IIN couples the entire image intothe HBE. However, splitting the input image into two enables moreoptimal coupling of the image into waveguide paths. In the followingdiscussion also consider monochrome implementations only. Initial designcalculation by the inventors show that the fundamental approach issound, meeting near diffraction limited performance across the field ina compact design form while including features such as projected pupil(20 mm, ahead of the grating coupling point), telecentricity, are-imaged stop and less than 2% geometric distortion. The designwavelength for the monochrome implementation of the IIN is 532 nm. Theresolution is matched to 1080p LCoS vertically (LCoS pixel pitch: 2.8micron; Nyquist frequency 178 lp/mm.). Note that a feature of the designis that IIN/HBE can be located on the same side of the waveguide as theeye without compromising grating reciprocity.

Light is projected from each LCoS at F/2.8(focal length: 5.65 mm.) toprovide a FOV of 26° H×30° V. Light enters the HBE-I grating at anglesfrom 0° to +26°. The IIN is inclined at angle of 13°. For a typical 0°(input)/52° (in glass) grating, angular this increases angular bandwidthis increased by approximately 20%. Note that red colored rays in FIG. 13strike the HBE-I at 0°, and diffract into TIR. The green colored raysstrike the SBG at approximately 26°/n where the refractive index n is1.592 (polycarbonate). Note that S-BAL25 glass which has a very similarrefractive index to polycarbonate can be used for prototyping. Thedesign can be extended to two and three colors. Initial results showthat the fundamental approach is sound, meeting near diffraction limitedperformance across the field in a compact design form while includingfeatures such as projected pupil (20 mm. ahead of the grating couplingpoint), telecentricity, a re-imaged stop and less than 2% geometricdistortion. The design wavelength is 532 nm. The resolution is matchedto 1080p LCoS vertically (LCoS pixel pitch: 2.8 micron; Nyquistfrequency 178 lp/mm.). Preliminary specifications for the IIN areprovided in Table 2.

TABLE 2 Input Image Node (IIN) Specification. Input Image Node (IIN)Optical Specification Glass Polycarbonate (can use S-BAL25 glass forprototyping). Image Format LCoS aspect ratio of 3:2 in portrait. PupilDiameter 2 mm. projected pupil (20 mm. ahead of the grating couplingpoint) LCoS Projection Lens FOV of 26° H × 30° V; F/2.8; focal length5.65 mm. Color Monochrome 532 nm. (proof-of-design); extendable tocolor. Resolution Near diffraction limited across the field; matched to1080p LCoS vertical pixel pitch: 2.8 micron; Nyquist frequency 178 linepair/mm.). Geometric Distortion <2%. Telecentricity Fully telecentric.

In one embodiment there is no hard physical stop in the projectionoptics but instead a projected stop is provided. The benefits of aprojected stop are decreased waveguide thickness. In one embodiment thestop is projected midway up the HBE to minimize aperture diameter withinthe waveguides, and hence minimizing the aperture width of the DigiLenswaveguide coupler.

In one embodiment a graduated reflection profile underneath the SBGlayer is used to control (or assist) with grating DE variation alonglength of the DigiLens waveguides. This normally achieved in SBG gratingusing index modulation. This may offer advantages the HBE where a lowpercentage of light is out coupled in 1^(st) bounce, but high percentageis coupled out at the other end of the waveguide.

The volume of the IIN design is currently×20×40 cubic mm. However, itwill be clear from consideration of the drawings that there are manydifferent ways to reduce the overall volume of the IIN design. Forexample the refractive elements of the design such as the bird bathmirror and the projection lens system could be replaced by diffractiveoptical elements. An SBG waveguide could be introduced in front of themicrodisplay to provide a polarizing grating beamsplitter forilluminating the microdisplay and allowing polarization rotatedreflected light to proceed through said wave guided towards the curvedmirror.

In one embodiment illustrated in FIGS. 21-22 a HBE 281 comprises acoupling grating 283 (e.g., the HBE-I of the earlier embodiments) at itsinput end and an output grating (e.g., the HBE-O) comprising alternatingSBG stripes of two different prescription 284, 285 inclined at typically45 degrees. Although the stripes are shown as equally spaced their sizeand spacing may be varied for better illumination and image samplingcontrol. However, making the strips too narrow may degrade the systemMTF. In general, the stripe geometry may need careful optimisation. FIG.22 shows the HBE integrated in a display with a DigiLens 287 comprisingDIGI-I 288 and DIGI-O 289 and an IIN 286. The ray paths from the IIN areindicated by 1105-1110 where the TIR paths in the HBE and DIGI-O areindicated by 1107,1109. The light coupled output of the HBE into theDigiLens (DIGI-I) is indicated by 1108. The output light from thedisplay from the DIGI-O is indicated by 1110.

FIG. 80 provides a transparent waveguide display in another embodiment.In this embodiment of the waveguide 8001, the amplitude of therefractive index modulation in at least one of the input grating 8003and output grating 8002 varies by a small amount along the x direction.In this embodiment, the input and output grating pitches should beidentical to satisfy the reciprocity requirement. The index modulationmay also be varied in the z direction. In one instance, the design ofthis embodiment may help control the output light homogeneity. Theprinciples illustrated in this embodiment may be applied to the verticaland horizontal beam expanders. In one instance, where switchablegratings are used the index modulation may be time-varied to adapt totemporal variations in the input image content.

In any of the embodiments of the invention efficient waveguiding mayneed that the TIR beams are confined between low index media. Air gapsare difficult to fabricate and maintain while the refractive indices ofcurrently available low index materials such as Magnesium Fluoride(1.46) and Silicon Dioxide (1.39) are much too high to meet the tightTIR angle constraints needed in full color implementations of the HMD.The proposed solution is to use nanoporous (Mesoporous Silicon)materials. Nanoporous materials (e.g., mesoporous Silicon) are currentlybeing used in many optical applications including anti reflectioncoatings and planar optical waveguides. Their high porosity enables thefabrication of high-quality low-dielectric constant thin films.Nanoporous materials can be fabricated in thin layers in a singlecoating step. To achieve very low, near unity, index the porosities needto be very high, approaching 95%. High transparency and low index can beachieved simultaneously with these films. Since they are highlyefficient at absorbing water they must be carefully sealed againstmoisture. The best approach may be to seal the passive gratings, HWP andmaterial together. SBG Labs is also investigating the potential role ofnanoporous materials as high refractive index media. This would increasethe range of TIR angles that can be sustained in our waveguides withpotential for increasing the horizontal FOV from 40° to around 45°.Nanoporous materials are currently being used in many opticalapplications including anti reflection coatings and planar opticalwaveguides. It is reasonable to assume therefore that the technologywill be accessible for our project. The manufacturing process should betranslatable to specification desired. Nanoporous materials can befabricated in single coating step. Alternatively graded index multilayer architectures can be used. SBG Labs is also investigating thepotential role of nanoporous materials as high refractive index media.This would increase the range of TIR angles that can be sustained in ourwaveguides. In summary the chief benefits are a monolithic structurewill provide greater mechanical stability and durability and better beamconfinement leading to higher FOV.

Embodiments for Automotive HUDs

As already discussed, the invention may be used in many differentapplications. Some embodiments of the invention directed specifically atautomobile HUDs will be discussed in the following paragraphs.

FIG. 23 is a is a schematic side elevation view of an in car HUD 300with more than one exit pupil integrating DigiLens waveguides for thedriver 301 and passenger 302. The display may be based on any of theembodiments described above. The DigiLens elements are integrated withina common waveguide structure with the input imagery being produced by anIIN 303 as described above. The ray paths to the driver exit pupil areindicated by the rays 1110-1113 with the pupil indicated by 1113. Theray paths to the passenger exit pupil are indicated by the rays1114-1116 with the pupil indicated by 1117.

FIG. 25 is a schematic side elevation view of a show an embodiment ofthe invention similar to the one of FIG. 23 the exit pupil of the driverdisplay is tiled using multiple overlapping DigiLens elements 305,306 toprovide the abutting exit pupils 1120,1121. Ray paths to the pupil areindicated by 1118-1120.

FIG. 25 is a schematic side elevation view of a further embodiment ofthe invention based on the one of FIG. 23 in which overlapping DigiLenselements 311,312 are used to tile the FOV as indicated by 1131 with theabutting field of view tiles 1132 while providing a common exit pupil1133.

In one embodiment waveguide a DigiLens may be used to form a pixelatedcollimated image. A simple classical analogue of such a display which isshown in the schematic side elevation view of FIG. 26 comprises apixelated display panel 321 located at the focal surface of acollimating lens 320. A waveguide holographic version of this display isshown in FIG. 27 . The input image is provided by a pixelated SBG 323comprising a two dimensional array of switchable elements such as theone labelled by 324. Each element diffracts incident collimated lightinto a TIR path within the waveguide. The SBG array is illuminated bycollimated light indicated by 1142,1143 from an external source which isnot shown. The pixel 324 is illuminated by the collimated light 1145.Advantageously, the SBG pixels will have diffusing characteristics. TheDigiLens elements 325,326 are not simple planar gratings such as theones described so far but have optical power such that light originatingat points on the surface of the SBG array is collimated to provideswitchable fields of view (FOV tiles) bounded by the rays 1146,1147 forviewing through a pupil 1148. Hence the DigiLens elements provided adiffractive analogue of the lens in FIG. 27A. The DigiLens elements areconfigure to tile the FOV as in the embodiment of FIG. 25 .

In one embodiment a DigiLens as described above may be configured toprovide a mirror. FIG. 28 shows how a rear view mirror for automotiveapplications can be provided using transmission SBG 333 sandwichedbetween the substrates 330,331 and a mirror coating 332 overlaying theDigiLens. The SBG diffraction angles are designed such that lightincident on the DigiLens following the path labelled by R1,R2 isdiffracted by the SBG layer and reflected at the mirror layer while thatthe reflected path labelled R3,R4 leading to the drivers eyebox is offBragg or, in other words, falls outside the angular range fordiffraction by the SBG. The light paths are also labelled by thenumerals 1151-1154. It should be apparent from consideration of FIG. 28that the DigiLens can be configured to provide a range of differentreflection angles by a suitable choice of grating prescription.

FIG. 29 shows a HUD 335 for relaying an external image to a viewingpupil near to the driver. Light 1155 from the external image sourceenters the waveguide via the DigiLens 338 undergoes TIR as indicated by1156-1157 and is coupled out of the waveguide towards the viewer aslight 1158 by DigiLens 336. This embodiment may be useful for viewingblind spots. The apparatus of FIG. 29 further comprises a beam splitterlayer 337 which, by splitting incident TIR light into multiple paths,can improve homogeneity and minimize gaps in the output illumination.This principle may be applied to any of the other embodiments of theinvention.

FIG. 30 is a front elevation view of a car HUD 340 comprising a IIN 341and a DigiLens 342 according to the principles of the invention andfurther comprises a structured light source 343 emitting infrared light1159 and detector 344 for detecting return infrared light 1159 forsensing driver hand movements for display control.

FIG. 31 is an embodiment of the invention similar to that of FIG. 26 inwhich the information contained in the field of view tiles (FOV Tile1,2) is presented at different ranges indicated by D1,D2. Image lightfrom the TIN 351 is converted into wave guided light 1170. The DigiLens354 forms an image at range D2 with an FOV of 1174 centered on thedirection 1173 and the DigiLens 355 forms an image at D1 with FOV of1171 centered on the direction 1172. The two FOV tiles are viewedthrough the pupil 1175.

In most applications of the invention the preferred light source is alaser particular where tight constraints on collimating and waveguideconfinement need to be met. However, the invention may, with somemodifications, be applied using LEDs and other relatively narrow bandincoherent light source. FIG. 32A illustrates an embodiment of theinvention in which SBGs are used to compensate for the spectralbandwidth of RGB LEDs in a color HUD. The red LED illustrated has a peakoutput at 639 nm. and FWHM bandwidth defined at the wavelengths 634 nm.and 644 nm. as shown in FIG. 328 . The DigiLens comprises a stack of RGBdiffracting layers, each layer comprising input and output gratings. Ineach layer the SBGs are recorded to provide peak diffraction efficiencyvs. wavelength characteristics (along the waveguide) shifted by smallincrements from the peak wavelength as indicated in the Table in FIG.32C. The techniques for recording at Bragg grating that provides awavelength shift in play back, which normally involve control of theconstruction wavelengths and recording angles, are well known to thoseskilled in the art of holography. The RGB SBG layers are switchedsequentially and synchronously with the RGB LEDs. As shown in FIG. 32Athe color display 360 comprises red green and blue DigiLens waveguideslabelled by the symbols R,G,B and comprise DIGI-I gratings indicated by360R,360G,360B and DIGI-O gratings indicated by 361R,361G,361B, red,green and blue light sources 362R,362G,362B providing light 1180 amicrodisplay 363 a bema expander comprising the diffractive orholographic lenses 364,365 for providing collimated light 1182. Afterbeing coupled into the DigiLens elements the light undergoes TIR asdescribed above and represented by 1183 and is diffracted out of theDigiLens as the red, green, blue light 1184R,1184G,1184B.

In one embodiment illustrated in FIGS. 33-34 the DigiLens is combinedwith a windscreen. A DigiLens is eminently suitably for such animplementation as it can be designed to operate in a curved waveguideand can be built up from very thin layers using substrates as thin as100 microns sandwiching SBG layers of thickness 1.8-3 microns. TheDigiLenses are separated by thin layers of mesoporous materials of thetype described above. It should also be noted that typical carwindscreens have radii of curvature typically of several thousand mmwhich does not present a great challenge for maintaining waveguiding.

In one embodiment the DigiLens is formed as a flexible layer 371 thatcan be bonded onto an existing windscreen 370 to the inner or exteriorsurfaces as shown in FIG. 33 . The IIN 372 would typically be locatedbelow the dashboard.

Alternatively, the DigiLens layers can be integrated within a windscreenas part of the screen fabrication process as shown in FIG. 34 .Typically, a windscreen comprises an outer toughened glass layer 373;two or more layers of PVB 374,376 for UV blocking and an inner toughenedGlass layer 377. The DigiLens 375 would be sandwiched by two of the PVBlayers. In one embodiment the windscreen integrated DigiLens fabricationprocesses includes the steps of spraying RMLCM onto PVB film andsandwiching it with a second PVB film prior to recording an SBG in aholographic recording step.

In one embodiment shown in FIG. 35 there is provided a transparentdisplay comprising the waveguide components DIGI-I. DIGI-O HBE-I, HBE-Oand an IIN similar to the ones described above. However in this caseeach said waveguide component comprises a single SBG layer. Note thatonly the SBG layers in the above waveguides are illustrated with thesubstrates and electrodes being omitted. The ray paths from the IIN areindicated by 1200-1203 include the TIR path in the HBE 1201, theextraction of the expanded beam from the HBE 1202, TIR within theDigiLens 1203 and extraction of light for display 1204 from theDigiLens. The components are also reference by the numerals 390-394.

In one embodiment shown in FIG. 36 them is provided a transparentdisplay DIGI-O, HBE-I and an IIN similar to the ones described above.The two waveguide components HBE-O and DIGI-I are combined into a singlewaveguide component labelled HBE-O/DIGI-I that performs the dualfunctions of beams expansion and beam turning. The advantage of thisarrangement is the change in beam direction is accomplish without theneed to exit and then re-enter the waveguide gratis as tor example inFIG. 35 and most of the other embodiments. This may need that the gratinin the DIGI-I is slanted in the plane of the drawing, in most cases ofpractical interest at an angle of 45 degrees. Hence within the DIGI-Iwaveguide the TIR path is rotate though 90 degrees and proceed into theDIGI-O. The ray paths from the IIN are indicated by 1205-1208. Thecomponents of the displays are also reference by the numerals 400-403.

It should be apparent from the drawing and early description that inthis case and indeed in most of the embodiment of the inventions theHBE-I and HBE-O and the DIGI-I and DIGI-O may be implemented in a singleSBG layer. However while this reduce the number of layers overall thereis the penalty the overall size of the display will increase. The widthsof the HBE and DIGI-O will to a large extend be terminated by themicrodisplay dimensions and the field of view needed. While in HUD andHDD applications the space needed to implement the DIGI-O and HBEcomponents are small relative to the size of the DIGI-O, the trade-offbecomes more challenging in HMD and near eye applications which will usesimilar size microdisplays and will have more demanding FOV demand,which will further increase the relative widths of the HBE and DIG-Iwaveguides.

In a further embodiment of the invention directed at a color displayillustrated in FIG. 37 the DigiLens comprises separate red green andblue waveguides. Advantageously the waveguides are optically isolatedfrom each other which may need that they are air separated or separatedby a thin layers of low index film. The preferred option is to usenanoporous materials. The DigiLens waveguides are shown in FIG. 37 thelayers being referenced by numerals 1-3 in the DIGI-I and DIGI-O labels(the waveguide components also reference by numerals 415-420. The IINand HBE components are not shown in detail but are simply indicated by421.

Typically the DigiLens will be inclined at a rake angle of around 10° toenable the most favorable diffraction angles within the waveguides asshown in FIG. 1 The IIN and HBE may be on the side of the displaynearest the eye 422 as showing in the drawing. However it should beapparent that many other configurations are possible. A ray path to theeye is indicated by the rays 1210-1212. FIG. 3 is a three dimension viewof the same display shown the three DigiLens lens

FIG. 38 shows a color display based on the embodiment of FIG. 35 . Thisimplementation has three DigiLens layers 410-412, one for each color,with the DIGI-I and HBE-I being combined in turning/beam expansiongratings such as the ones indicated by 436 red green and blue ray pathsfrom the IIN are indicated by 1220-1223 with red, green and blue beinglabeled by the symbols R,G,B. Note that the HBE-I component which is notillustrated will be similar to the one shown in FIG. 35 but withseparate layers for red, green a blue.

In auto HMDs fields of view are relatively modest compared with those ofHMDs a field of view of 10 degree vertical by 25 deg. horizontal isconsidered to be a long term goal. Such angular content can easily behandled by a waveguide using a single layer SBG. However since there isinterest in color this will make further demands on the informationhandling capacity of the waveguides In one embodiment shown in FIG. 39there is provide a passive single SBG layer DigiLens which waveguidesand diffracts at least two colors using holographic multiplexing.Multiplexing is the ability to record multiple Bragg gratings in thesame layer. Firstly it can be used to produce improved angular profilesby combining two gratings of similar prescription to extend thediffraction efficiency angular bandwidth and giving better luminanceuniformity and color balance across the exit pupil and field of view.Secondly multiplexing may be used to encode two distinct diffractionprescriptions which may be design to project light into distinct fieldof regions or diffract light of two different wavelengths into a givefield of view region. Multiplexing also offers the significant benefitof reducing the number of layers in the waveguiding structure. Care isneeded to ensure that there is no competition between gratings duringrecording leading to unequal Des and cross talk between gratings inplayback. Tile apparatus of FIG. 39 is similar to that of FIG. 35differing mainly in that the DIGI-O component is now a multiplexedgrating. The components are also referenced by the numerals 470-474 andray paths from the IIN through the HBE and DigiLens by numerals1240-1243. Advantageously, the Horizontal Beam Expander (HBE) would runalong bottom edge of the DigiLens® to enable the IIN to be hidden belowthe dashboard. Typically such a HUD would have an eyebox of 145 mm.horizontal by 80 mm. vertical at an eye relief of 800-900 mm. The targetfield of view would be 10°-15° vertical×10°-15° horizontal. Thewindscreen would have a transparency greater than 75%. Typicallywindscreens have curvature of Horizontal: 3000 mm.; Vertical 9000 mm.The IIN could be accommodated within a volume of 1-2 litres.

The integration of the DigiLens into the windscreen is shown in FIG. 40. The stack of layers in the windscreen comprises Glass (External) 1.60mm.: PVB (3 layers): 0.8 mm.; Glass (Internal): 2.0 mm. TOTAL 4.4 mm.The SBG layer would comprise an SBG of 5 microns sandwiched bysubstrates of 50-100 microns, the SBG cell sandwiched by low indexlayers of 0.5-2 microns thickness. A UV glue gasket line limits the SBGmaterial fill, bounding the effective area of the display. Gaps betweengrating areas are filled with clear acrylic and UV-cured for fulltransparency. This multilayer architecture allows very strong laminationand does not rely at all on the SBG film for lamination strength. FIG.41 is a flow chart illustrating the implementation of the embodiment ofFIG. 39 in an automobile HUD.

The color multiplexing used in the embodiment of FIG. 39 can be providedin two ways: firstly using two multiple gratings: for example, red andblue/green diffracting; or secondly using three multiplexed gratings;i.e. using multiplexed red green and blue gratings. The design problemis to determine the optimal prescription using two or more multiplexedgratings for a vertical (or more precisely tangential plane) field of 10degrees. The design variables are Bragg wavelengths and TIR angles foreach color; DE profiles (i.e., grating thickness and modulation. Sincethe DigiLens will be bonded to or in some cases embedded with awindscreen the waveguides will be sandwiched by low index material(unless air gaps can be tolerated). A nanoporous material layerrefractive index of 1.2 has been assumed. For an SBG index of 1.52 thiscorresponds to a minimum TIR angle of 52 deg. In designing a multiplexedgrating special attention may be needed to achieve the shape of the DEprofile is important and, in general, narrower DE profiles are preferredto avoid crosstalk between the colors. The desired DE profiles can beachieved by optimising the thickness and refractive modulation of thegratings. Simulated diffraction efficiency (DE) versus TIR angleprofiles based on the Kogelnik diffraction theory for typical red, greenand blue diffracting gratings multiplexed in a single SBG layer areshown in FIGS. 42A-42B. The curves represent the DE profiles for each ofthe three multiplexed gratings. The rectangular regions represent theapproximate RGB TIR angle ranges. FIG. 42A shows the case where themultiplexed grating is illuminated with red light. FIG. 42B shows theease where green or blue light illuminates the grating. Note that thecurves shift owing to the angles satisfying the Bragg condition shift asthe illumination wavelength changes. Since the TIR range for the bluegreen band is below the TIR range for red, there is no cross talkbetween the two color bands.

FIGS. 43-44 illustrated a further embodiment of the invention comprisinga IIN, two HBE components each comprising a HBE-I and HBE-O; twoDigiLens components each comprising a DIGI-I, DIGI-O and beam splitter(B/S) the DIG-I sandwich a half wave film (HWF). FIG. 43 shows a threedimension view. FIG. 44 is a cross sectional view of the DigiLens onlyshowing the ray paths. The HBE-O and DIGI-O gratings multiplex red andblue-green diffracting gratings as indicated by the solid and dashedrays respectively in FIG. 44 . The red rays in the rear DigiLenswaveguide 460 follow the path 1231,1232,1239. The blue-green rays in thesame waveguide follow the ray path 1230,1233,1239. The ray paths in theforward waveguide are labelled by the numerals 1234-1237. The rear andforward waveguide components are referenced by numeral 1,2 respectivelyand are also referenced by the numerals 440-447. The HBE comprises twowaveguiding element (i.e., two switchable SBG layers separated bysuitable substrates) each supporting 20 degrees horizontal×22.5 degreesvertically. The HBEs are switchable. Each HBE waveguide is separatedfrom the other by a low index layer. The IIN would use a HD1366×768(0.37″ diagonal LCoS microdisplay such as the one supplied byHimax Inc. An alterative microdisplay is the Syndiant 720P (0.37″display) supplied by Syndiant Inc. This would give a resolution of 1.76are min/pixel (over the full 40 degree×22.5 degree FOV). Each gratingprovides approximately 10 degrees in air (equivalent to about 6.5degrees in glass) of angle width for a single color. There is a slightbias of angular space due to the HBE rake angle (on side entry of IIN)which causes the fields to be slightly unequal e.g. 18 degrees and 22degrees. Note that switching of gratings adjacent in angular spaceensures cross coupling will not occur between adjacent gratings. Anglesin each waveguide are minimized. Green is arranged close to TIR anglewith a small tolerance margin (e.g. +0.5 to 1.0 degrees). Each waveguideis narrowband blue mirror coated to extend the waveguide angular range.The light output from each HBE into the DigiLens is: −20 to 0 degreeshorizontal (approximately)×±11.25 degrees vertical; and 0 to +20 degreesg horizontal (approximately)×±11.25 degrees vertical. This gives acomposite field is −20 to +20 degrees horizontal×±15 degrees (i.e., 40degrees×22.5 degrees). The preferred material is polycarbonate(n=1.585), or equivalent. The substrate thickness is 0.5 mm. Each cellis 1.0 mm thick, so that each BIE waveguide is 2.0 mm thick. Both HBEwaveguides are 4.0 mm thick with a small air or low index material gap.The HBEs would be multiplexed for color as discussed above. The HBEdimensions are: 70 mm in width×11 mm in height. Note that the heightincludes a 1 mm margin on the vertical edges. The IIN used in theembodiment of FIGS. 43-44 has a focal length of 1.3 mm. The microdisplay(LCoS) pixel pitch is 6.0 micron. The optics F-number is 4.6. Bymatching the Airy disc to LCoS pixel pitch, approximately 75% contrastat the Nyquist limit is obtained. The aperture of the IIN is 2.4 mm. Theoptical design of FIGS. 43-44 may need two DigiLens waveguides (i.e.,two SBG layers separated by suitable substrates), each supporting 11.25degrees vertical×40 degrees horizontal. The input DIGI-I elements wouldbe switchable. The DIGI-O elements would be passive. Both input andoutput gratings would be multiplexed red and green-blue gratings asdescribed earlier. The dimension of DigiLens substrate is 50 mm width×61mm height. In one embodiment the input gratings are switchable, and notmultiplexed, that is four switchable input gratings are used in thiscase, importantly, the input and output prescriptions are reciprocal.The input light must be P-polarized (since the DigiLens only diffractsP-polarized light). Each waveguide has a QW film down full length: Thispermits a 2× thinner waveguide. TIR reflections are rotated by a halfwave (owing to the double pass through the QW film). Ray extraction fromthe waveguide thus occurs only at every other interaction with SBG. Theoutput of the rear DigiLens (i.e., the one nearest the IIN as shown inFIGS. 43-44 ) experiences two QW film interactions, rotates to S andtherefore does not interact with the forward DigiLens. Hence the outputfrom the display is mixed P and S polarized light.

In one embodiment illustrated in FIGS. 45-52 there is provided atransparent display based on completely passive HBE and DigiLenswaveguide components. As will be explained in the following descriptionthis embodiment uses a HBE-I configured for sampling the aperture and/orFOV of the image light from the IIN. This embodiment will now bedescribed with reference to a 52 degree×30 degree FOV monochrome displaysuch as may be used to provide a HMD. However, it should be apparentfrom the following description and drawings that this embodiment maywith the benefit of the teachings presented herein be used to provide awide range of different FOVs.

An Embodiment Using all Passive SBGS

FIG. 45 is a three dimensional illustration of an embodiment of adisplay in one embodiment in which there are provided three HBEwaveguides and three DigiLens waveguides. The HBE waveguides areindicated by numerals 541 and the DigiLens waveguides by 542 with theHBE waveguides being labelled by symbols W-Z as before and the DigiLenswaveguides by symbols P-R. Each HBE waveguides contains a HBE-I andHBE-O also referenced by 543,544 respectively and each DigiLens containsa DIGI-I and a DIGI-O also referenced by 545,546 respectively. The IINis indicated by 394. A ray path from the IIN through the HBE waveguide541Z and DigiLens waveguide 542R is shown using the rays 1330-1334. Notethat the pupil; has been combined before the DIGI-I so that the full 52degree horizontal×30 degrees vertical FOV exist at this stage.

One embodiment is related to a projected stop. FIG. 46 is a schematicside elevation view showing the formation of a projected stop by theIIN. Light from an image pixels at the center and edges of the imagedisplayed on the microdisplay 500 passes through a stop 502 to providethe marginal beams 1300,1302 and the on axis beam 1301. The projectionlens 501 collimates this light to provide the collimated beams 1303-1305which overlap at the projected stop 1306. Light from other points in theinput is similarly converged into the projected stop. Note that theprojection optics has been simplified in FIG. 46 and in practice theprojection lens will be a multi-element system as discussed earlier. Aswill also be appreciated by those skilled in the art of optical designthe need for a physical stop 502 be minimized (or even eliminated) bylimiting the numerical aperture (NA) at the microdisplay by suitabledesign of the illumination delivery optics. It should be noted that thedistance from the lens to the projected stop is very large compared tothe thickness of the HBE stack.

FIG. 47 is a schematic plan view illustrating the coupling of light fromthe IIN into the HBE waveguide 503. The input grating or HBE-I comprisesthe two gratings 504,505 having a small separation along the waveguidepropagation axis. Importantly, the two gratings have substantiallynon-overlapping DE versus angle characteristics. The HBE-O grating islocated further down the waveguide as indicated by 506. In contrast tothe input gratings the output grating multiplexed two different gratingprescriptions as will be explained below. The advantage of this couplingscheme is that it enables mapping of the angular content of the inputlight to defined gratings areas 504,505 allowing the input gratings(HBE-I) of the HBE to be separated along the waveguide optical axis.This effect is accentuated by having small entrance pupil diameters. Forsimplicity illustrated herein are just two ray paths through thewaveguide. In the first path an incident rays 1307 is diffracted into afirst TIR path 1309 by the first grating element 504 of the HBE-I. Inthe second path incident ray 1308 is diffracted into a second TIR path1308 by the second grating element 505.

The interaction of the beam with the gratings is illustrated in moredetail in FIG. 48 which shows a portion of the HBE waveguide containingthe input gratings of FIG. 47 . The incident ray 1307 is diffracted bythe first grating 504 into the ray path 1311. After the first TIRreflection the reflected ray 1312 is off Bragg and therefore passesthrough the first grating without deviation. After the second reflectionthe ray just skirts the trailing edge of the first grating avoidingdiffraction. TIR now proceeds to the next grating. After the thirdreflection the ray 1314 skirts the leading edge of the second grating.After the fourth reflection the resulting ray 1315 is off Bragg.Finally, after a fifth reflection the ray 1316 skirts the edge trailingof the second grating and proceeds to the HBE-O. Clearly this simplisticexplanation only applies to the chief ray and does not adequatelydescribe the behavior of a divergent beam which will result in unwanteddiffractions of rays that intercept the gratings. However, the narrowbeam angles within the waveguide will reduce the impact of unwantedbeam-grating intersections.

FIG. 49 is a schematic cross-sectional view of the four layer HBE shownin FIG. 45 . Following the above description each waveguide layercomprises a HBE-I gratings comprising two separate gratings and a HBE-Ocomprise a multiplexed grating. The waveguides may be separated by airgaps or preferably by thin layers of nanoporous low index material. Theinput gratings are labelled by the numerals 511,512 and the outputgrating by the numerals 510 with the waveguide layers being referencedby the symbols W, X, Y, and Z. The drawing shows input rays from theprojection lens 514 interacting with each of the input gratings in eachlayer. Note that the output light from waveguide layer W may interactwith one or more of layers X-Z. However, as already discussed theproperty of grating reciprocity maintains the beam angle for any suchinteractions. The result beam shifts resulting from such interlayerinteractions may provide beamsplitter or homogenizer. For example,consider the input ray 1320. This ray is diffracted by the first grating511W into a TIR path 1321 which propagates through the second grating512W without deviation and continues its TIR path 1322. At the outputgrating 510W a portion of this light is diffracted into the ray 1323 inwaveguide 510X. On entering the second waveguide a portion of this lightis diffracted into the TIR path 1324 the remainder proceeding in thedirection 1323. Further interactions with the grating 510Y are indicatedby the rays 1325 and 1326.

FIG. 50 is a table showing the pairing oft multiplexed gratings used ineach waveguide layer of the embodiment of FIG. 48 . Each pair ofgratings must have DE versus angle bandwidths with sufficient separationto avoid appreciable overlap. Since the gratings will be lossy thingratings characterized by low DE peaks and wide angular bandwidths thisseparation needs to be quite large. FIG. 51 is a chart a chart showingone possible scheme for overlapping the DE versus angle profiles in theembodiment of FIG. 45 . Each pair of gratings corresponds to a 26 degreechief ray angular separation. It is assumed that the total horizontalFOV is 52 degrees. For a thin (low peak DE, high ABW) grating the ABW isapproximately 13 degrees. Assuming that the input HBE gratings have ABWsof 6.5 degrees referring to the angle ranges: −6.6 degrees to 0 degreesas A and the angle range: 0 degrees to 6.5 degrees as B. It isrecognized that A and B should not be used in the same waveguide owingto: firstly. DE versus angle overlap and secondly, lack of reciprocity.The grating separations are based on the FWHM. However, other measuresmay provide better optimization of the overall illumination profile. Forexample the grating overlaps could be at the 30% of peak angles.Assuming FWHM this gives 8 gratings to span a FOV of 52 degrees (i.e.,8×65 degrees).

Achieving high illumination uniformity is an important issue in SGOdesign. The chief design parameters are thickness and index modulation.As discussed earlier, applying a small wedge angle to the grating layerallows the peak DE and angular bandwidth to be tuned along thewaveguide. However, the light remaining in the waveguide at the end ofthe propagation path will tend to accumulate at larger angles leading touneven output illumination. At present the inventors believe that thiseffect may not be very significant. A more sophisticated strategy incases where very tight illumination control is needed is to shape thegrating layer by applying tilt in two directions. As a furtherrefinement the surfaces of the grating could be curved. Since the neededgrating thickness variations are very tiny typically 1-2 microns acrossthe waveguide the effect on collimation and MTF are negligible in mostimplementations. FIG. 51 is a three dimensional view of an SBG grating530 characterized by two orthogonal slants. The four different cornerheights with respect the plane of the waveguide resulting from theorthogonal slants are indicated by 531-532. However, preliminaryray-tracing analysis by the inventors has yielded some evidence that theoptimal DE profile needed within the waveguide is a one dimensionalfunction and is a function of the length of the waveguide not the numberof bounces of the beam within the waveguide. The ray angle displacements(i.e., ±angles) around the chief ray) appear to need the same gratingthickness profile as the chief ray (as a function of the length of thewaveguide). This suggests that a 2D profile may not be needed foroptimum uniformity. Note that to first order DE must scale as thereciprocal of the difference between the waveguide length and thedistance traveled by the beam.

FIGS. 53A-53C are schematic illustrations of a three operational statesof the HBE in one embodiment. A portion of the waveguide 551 and theprojection of the IIN projection lens 550 is shown in each cased withthree collimate beam portions indicated by 1341-1342. The shaded areascorrespond to the HBE cross sections seen by each DigiLens waveguide foreach of the three separate 10 degree vertical FOV bands coupled into oneof the DigiLens waveguides to give the 30 degree total vertical FOV. Theshaded areas in FIGS. 53A-53C also show the portion of the DIGI-I thataccepts light from the HBE-O for the three vertical FOV regions. Only ⅓of the aperture is collected by DIGI-I in each state. By keeping theeffective aperture low in this way it is ensured that the DigiLenswaveguides do not need a large thickness. It is estimated that the totalthickness needed for the three DigiLens waveguides using the aboveaperture method (as well as the polarization management mentionedearlier in the description) will result in an overall thickness of 6 mm.This assumes a rectangular exit pupil. If a circular pupil is acceptablethe need to achieve the extreme diagonal angles is removed resulting ina lower overall thickness.

Although all passive grating as described above is feasible inmonochrome it is likely that switching will need to be introduced toprovide full color to ensure that crosstalk is overcome.

It is important to note that a unique feature of the above embodiment isthe way in which the HBE selectively samples portions of the input imageeach portion being characterized by either angular field or spatialfield. This approach ensures that the waveguides in particular theDigiLens can be made very thin. This is a particularly important featurein HMDs and near eye. The process of sampling the input image describedabove should be distinguished from the process of switching the entireinput image into the waveguides used in FOV tiling, as disclosed inearlier filings.

Exemplary Features

In one embodiment optical power may be provided in at least one of thegratings used in the HBE and DigiLens waveguides. The recording of lensprescriptions in diffractive optical elements is well known. In thepresent invention this offers potential for fine tuning the focus of thedisplay without the need for auxiliary lenses. Such a lens could also beused for correcting aberrations. A switching hologram offers thepotential for a solid state (no moving parts) solution for providingimages at different ranges. This may need multiple layers. This wouldprovide an attractive feature in HUD and HDD applications. Recordingholographic lens of appreciable optical power may need careful attentionto overcoming off-axis holographic aberrations. The construction opticsare potentially more complicated but once a master hologram is made, thecontact copy process is the same as any other hologram.

In one embodiment a multilayer thin film coating is applied to areflection surface of the DigiLens waveguide is to enhance thewaveguiding range beyond the TIR range. Glasses and plastics do notallow the range of TIR angles need for large field of view. For exampleRays below 39° are not supported by polycarbonate TIR (refractive indexat 532 nm=1,585). The problem boils down to achieving a minimumwaveguide angle 30 degree angle which is well below critical angle forplastics and low cost glasses. The coating design problem is to achievea reflectivity coating prescription that achieves the neededreflectivity, without image degradation or loss of see-throughtransmission. The coating may need optimizing for wavelengthpolarisation, angle, scatter, Loses from absorption. One benefit ofapplying dielectric films on the substrate significantly reduces theamount of diffractive power needed, thereby improving grating angularbandwidth. The dielectric coating has the following characteristics: a)high reflectivity for S&P light for angles of 30° (in glass) to 39° (TIRangle), b) high transmission for 0°±13° (in glass) for P-light; equatesto 35°×20° corner fields in air; c) good angular control of reflectivitycan be achieved due to narrow band nature of sources; and d) high seethrough for non-laser notch wavelengths on all layers.

In one embodiment the input gratings in at least one of the HBE orDigiLens, i.e., the gratings HBE-I and DIGI-I, comprises stacks ofgratings of different k-vectors to provide DE versus anglescharacteristically distributed uniformly over the range of beam anglesto be transmitted through the HBE and DigiLens. Typically the gratingswill be separated by 200 micron (or even 100 micron) substrates) to keepthe stack as thin as possible. Ultimately, the inventors believe thatthe minimum practical grating stack thickness can be achieved using spincoating techniques.

In one embodiment an alternative scheme of multiplexed gratings ofdifferent k-vectors avoids the need for stacking.

Any of the above-described embodiments using laser illumination mayincorporate a despeckler device for eliminating laser speckle disposedat any point in the illumination path from the laser path to theeyeglass. Advantageously, the despeckler is an electro-optic device.Desirably, the despeckler is based on a HPDLC device.

In one embodiment the display incorporates a homogenizer that combinesbeam shaping and despeckling. Desirably, the SBG array used to performthe above functions are themselves diffractive waveguide devices thatcan be implemented seamlessly as additional SBG layers within one ormore of the waveguides or in the IIN. Despeckling is achieved by acombination of angular and phase diversity. Exemplary waveguidehomogenizers are disclosed in U.S. Provisional Patent Application No.61/796,795, entitled COMPACT LASER ILLUMINATOR INCORPORATING ADESPCKLER, with filing date of 20 Nov. 2012 and PCT Application No. US2008/001909, with International Filing Date: 22 Jul. 2008, entitledLASER ILLUMINATION DEVICE. It should be apparent from consideration ofthe drawings and descriptions that the invention offers severalopportunities for integrating such devices within the IIN and thewaveguide components of the display.

The invention provides a transparent display based on a DigiLens whichcomprises one or more passive SBG waveguides, each one projecting aportion of the vertical field of view or a specific color. The inventionis enabled by improvement in diffraction efficiency angular bandwidthobtained from using thinner SBGs and taking advantage of theintrinsically broad sagittal angular bandwidth of Bragg gratings. Theinvention can deliver a large field of view, for example 52°horizontal×30° vertical, without sacrificing any of the usual goals ofhigh transparency, high resolution, ultra compact (thin) form factor,light weight and generous eyebox. The waveguide components and compactinput image node are consistent with a small form factor, path to curvedvisor, and slim-line goals. With the benefit of expected improvements inangular bandwidth and optical efficiency, it is believed that the aboveFOV can be increased. The display modular design approach permits readygrowth from monochrome to color with no major architecture redesignbeing needed. Reasonable imaging performance can be delivered out to 1.0cyc/mr for both color and monochrome solutions. Optical performance inthe monochrome will exceed the 1.4 cyc/mr display pixel resolution. Thebest resolution will be achieved in monochrome, but good performance canbe achieved also for a color display. In a HMD the invention can providea 25 mm wide eye box at 25 mm, eye relief. This will permit 90% of thepopulation to be accommodated without any adjustment. Substrate guideddiffractive optics are used everywhere except in the Input Image Node(IIN). However, the refractive components can be replaced by diffractiveelements in future developments of the design to yield further formfactor compression, and manufacturing benefits. The IIN may be mountedabove or to the side of and at the front or rear of the DigiLens. Thisallows a range of ergonomic demands to be met while preserving coreDigiLens functionality. The design may need no partitioning ortessellation of the near eye DigiLens, eliminating the problems ofillumination ripple and discontinuities and scatter from electrodes. Atransparent display according to the principles of the invention willalso benefit from results from plastic SBG technology disclosed inUnited

In one embodiment, a holographic brightness enhancing film, or othernarrow band reflector, is affixed to one side of the display, thepurpose of which is to reflect the display illumination wavelength lightonly; the see-through display can be made invisible (and hence secure)in the opposite direction of view. Here the reflected displayillumination is effectively mirrored and therefore blocked in onedirection, making it ideal for transparent desktop display applicationsin customer or personal interview settings, common in bank or financialservices settings.

An important performance parameter in the present context is thesee-through transmission of the display. The variables that have animpact on transmission are the ITO coating (0.995), the AR coatings(0.99), and the absorption of the substrates and holographic layers.There will also be Fresnel losses at the interfaces between thewaveguides and the low-index bonding layers. The needed transmission forthe color display is >70%, with an objective of >90%. Assuming threewaveguides per display and two substrates per waveguide, the calculatedtransmission is 93%, meeting the stipulated objective. Note that it isour intent to implement this design using 100-micron glass substrates.With three waveguides and three substrates per waveguide (note: twoholographic layers may need three substrates), the total thickness ofthe display of the color display is still less than 1 mm. Thethicknesses of the holographic layers (including the coatings) arenegligible; each contributes only 4-5 microns to the overall thickness.Since weight is always an issue, this is an extremely important featureof our approach. When plastic is employed, the weight may be reducedeven further.

In the preferred embodiment, the SBGs operate in reverse mode such thatthey diffract when a voltage is applied and remain optically passive atall other times. The SBGs will be implemented as continuous SBG laminaseparated by thin (as thin as 100 microns) substrate layers as shown.Ultimately the design goal is to use plastic substrates withtransmissive conductive coatings (to replace ITO). Plastic SBGtechnology suitable for the present application is being developed in aparallel SBIR project. This is a planar monolithic design harnessing thefull assets of narrow band laser illumination with monolithicholographic optics.

The present invention has a suite of advantages ideally suited forsubstrate guided optics. First, component costs are greatly reduced. Theoptical complexity is contained in the various holographic opticalelements. Once the non-recurring engineering (NRE) associated withcreating a set of masters is complete, the replication costs arerelatively insignificant compared to the recurring material costsassociated with discrete refractive components. Second, assembly time isgreatly reduced. Not only is part count greatly reduced, but theassembly process is much faster. The planar structures can becost-effectively laminated together with very high optical precisionusing alignment fiducials. The touch labor is greatly reduced comparedto that of building a piece-part assembly to exacting standards. Third,the optical precision is much greater. One of the biggest challenges indesigning a new optical design is controlling the roll-up of toleranceson the piece parts, the mechanical housings, and the assembly procedure.With holographic optical elements (HOEs), “gold standards” can beassembled by senior engineers and this level of quality captured in theHOE masters during the NRE phase. Besides the fact that opticalalignment of the HOEs can be accomplished with great precision, theindividual HOEs are more tolerant of variations in alignment. Thus, theoverall yield of high quality devices is much higher. Lastly, size andweight are greatly reduced by this monolithic design, as is theruggedness of the entire subsystem.

It will be clear that in any of the above embodiments the eye lens andretina may be replaced by any type of imaging lens and a screen. Any ofthe above described embodiments of the invention may be used in eitherdirectly viewed or virtual image displays. Possible applications rangefrom miniature displays, such as those used in viewfinders, to largearea public information displays. The above-described embodiments may beused in applications where a transparent display is needed. For example,the invention may be used in applications where the displayed imagery issuperimposed on a background scene such as heads up displays andteleprompters. The invention may be used to provide a display devicethat is located at or near to an internal image plane of an opticalsystem. For example, any of the above described embodiments may be usedto provide a symbolic data display for a camera viewfinder in whichsymbol data is projected at an intermediate image plane and thenmagnified by a viewfinder eyepiece. It will be clear the invention maybe applied in biocular or monocular displays. The invention may also beused in a stereoscopic wearable display. Any of the above describedembodiments of the invention may be used in a rear projectiontelevision. The invention may be applied in avionic, industrial andmedical displays. There are also applications in entertainment,simulation, virtual reality, training systems and sport.

The preferred light source for the display is a laser which is the idealmatch to the diffractive optical elements and therefore unleashes thefull power of our HMD, HUD and HDD designs. In terms of brightness,resolution and color gamut, the invention could also in someapplications where optical performance is not very important to beapplied using LEDs.

Currently, SBGs for use in one embodiment are manufactured usingstand-off exposure. However, the processes can be readily converted to acontact-copy process. The IIN may be implemented as an assembly ofdiscrete refractive components. However, it is highly desirable toconvert the design to a laminated stack of planar holographic elements.With sufficient volume, such an implementation of the IIN would achievea very attractive price point. Advantageously, the substrates used inthe waveguide would be fabricated from ballistic-quality plasticsubstrates. Proof of feasibility has been established and processoptimization activities are underway. Replacing refractive componentswith laminated plastic-based planar holographic elements will makedisplays based on the invention inherently more rugged.

Further Embodiments

The following embodiments are directed at a display that uses a singlewaveguide HBE. This particular embodiment is presently considered to bemore suitable for lower field of view devices such as HUDs. However,recognizing that the size of the HBE is likely to be manageable, thereis no reason in theory why the same embodiment could not be used forhigher FOV applications such as HMDs. As will be discussed, theprerequisite is a non-projected pupil.

FIG. 54 illustrates projection schemes that do not result in a projectedpupil of the type discussed earlier in the description. The pupil may beformed inside the projection lens, as shown in FIG. 54A, or before theprojection lens using the aperture 562 as shown in FIG. 54B. Suchprojection schemes result in an angular diversity at some distance alongthe z axis or optical axis that is less than the full FOV. For exampleturning to FIG. 54A, if light from the source image points 1350A-1350Cis considered, which is collimated by the lens 560 into the collimatedray bundles 1351A-1351C, overlap exits between the bundles 1351A,1351Band between the bundles 1351B,1351C. From basic geometry the angulardiversity to first order is given by the projection lens exit pupilaperture D and the distance along the optical axis z.

FIG. 55 is a schematic illustration of the use of rolled k-vectorgratings to maximize the peak DE of in-couple light, (for example in theHBE-I grating). The k-vector roll for the grating elements 572A-572C inthe waveguide section 570 is indicated by the differing orientates ofthe k-vectors 573A-573C. The surface grating pitch P is constant alongthe length of the waveguide. The grating vectors are optimized todiffract the rays 1360A-1360, representing the chief rays and off-axisrays, with high diffraction efficiency. The use of rolled k-vectorsenables high efficiency input coupling into a grating, and also allowsthe beam spread angle to be optimized to minimize the thickness of thewaveguide; this may need balancing the waveguide thickness, the angularbandwidth of the input grating, and the spread of field angles at anygiven point on the input grating. The low angular response of gratingsas the k-vector is rolled (and surface pitch maintained) prevents outputcoupling, allowing the waveguide thickness to be minimized.

FIG. 56 is a schematic illustration showing the propagation of a typicalray through a waveguide section 580 containing rolled k-vector gratings.The dimension a separating the points X and Y is approximately twice thewaveguide thickness t for a 45 degree TIR angle in the waveguide. Thepropagation path is indicated by the rays 1370-1371. The point Xcorresponds to the point at which the ray 1370 couples into the grating.The chief ray angle may be needed to change by an amount greater than orequal to the FWHM angular bandwidth of the grating. At point Y the rayangle is off Bragg. Hence, reciprocal output coupling at Y is notobtained. The design aim is to ensure maximum input coupling at X and atthe same time to design the distance along z to minimize the angulardiversity such that the grating thickness can be minimized withoutreciprocally out-coupling at position Y.

FIGS. 57-59 show perspective view of an embodiment that uses k-vectorrolling to provide exit pupil expansion in orthogonal directions. FIG.57 is a plan view showing the HBE 590 and the VBE 591. FIG. 58 is aschematic side elevation view of the HBE and the VBE. FIG. 59 is anunfolded view of the HBE showing the beam propagation inside thewaveguide. A Cartesian XYZ coordinate system is provided for reference.The lossy output grating of the HBE is indicated by 592. The inputgrating is indicated by 593. The input and output gratings have commonsurface grating pitch. At Z=0 and Z=L along the HBE, it may be desiredto have high input coupling within the VBE. As the angular diversitydiminishes as the limit Z=L is approached (angular diversity scales as1/L), the grating thickness can be increased. This is beneficial as thebeam fill W at Z=L is much greater than at Z=0 so that light in-coupledto the VBE with thickness t will experience more grating interactions inwaveguide following in-coupling. A thicker grating reduces out-coupling.The angular diversity can be used to fine-tune the thickness of the VBE.

FIGS. 60-62 illustrate an apparatus for fabrication the waveguideillustrated in FIGS. 57-59 . Referring to FIG. 60 , all points along theray 1390A such as 600A-602C must have identical surface grating pitchesand parallel k-vectors. The k-vectors are rolled in planes orthogonal tothe z axis. FIG. 60 illustrates an apparatus for fabricating the HBEusing a contact copying process. FIG. 61A shows a cross section of theZ=L end of the HBE 590 with the wider end of a cone shaped lensoverlaid. FIG. 61B shows a plan view of the lens and FIG. 61C shows theZ=0 end of the HBE with the narrower end of the lens overlaid. Thegrating layer is indicated by 611 and the rolled grating is indicated bythe detail 612. The paths of the collimated recording beam incident onthe lens are indicated by 1391,1393. The convergent rays that are usedto form the rolled grating are indicated by 1392,1394. The lens isillustrated as a refractive element. In one embodiment a diffractivelens of equivalent prescription may be used. The cone-shaped lenscontact copying set-up may need careful attention to overcomeholographic off-axis aberrations, which may need to be compensated atthe recording stage.

FIG. 62 illustrates the generation of the conic section from a cone oftip 620 and base 621. The cone is shown in side view in FIG. 62A whichindicates the cut out optic and the cut line and in-front view in FIG.62B, which again indicates the cut out optic. A view of the cut outoptics projected from the base along the cut line is shown in FIG. 62C.The conic section is obtained by cutting parallel to cone edge. Raysparallel to the z-axis then remain non-deflected (i.e., with norefracted component) in the y-axis. The amount of deflection(refraction) in the x-axis is a function of the position the ray strikesthe y-axis.

FIGS. 63-66 illustrate the principles of k-vector rolling, FIG. 63 showsthe basic architecture of a waveguide according to the principle of theinvention. The waveguide 630 comprises a multiplicity of grating laminawhich can be grouped into input gratings 631 and output gratings 632. Ineach case associated with each Bragg grating there will be a surfacegrating as indicated by 633,634. Input image light is represented by thecollimated beams 1400-1402 where characters A, B indicate the extremerays of each beam. The corresponding output image light is representedby three collimated beams 1410-1412 with characters A, B againindicating the extreme rays. The output beam has a greatly expandedpupil owing to the extraction of light along the waveguide as discussedabove. Typically the output gratings will be much longer, extending overmost of the length of the waveguide. The DIGI-I, HBE-I gratingsdiscussed in relation to certain embodiments are examples of inputgratings while DIG-O, HBE-O gratings are examples of output gratings, ineach group multiple gratings may be stacked or disposed in a layeradjacent each other. FIG. 64 is an illustration of a waveguide in whichthe input gratings 635A-635C are stacked. Each grating has a uniquek-vector 636A-636C. The k-vector 636A is designed to give highdiffraction efficiency for a field of view centered on the beamdirection 1401. The k-vectors 6361, 636C are optimized for highdiffraction efficiency around the incident beam directions 1400, 1402,respectively. Thus the input image is sampled into a plurality ofangular intervals. Each angular interval is associated with an effectiveexit pupil that is a fraction of the full pupil.

In the embodiment as shown in FIG. 65 , the input gratings are disposedadjacent to each other along the waveguide propagation direction. Thewaveguides are indicated by 637A-637C and the k-vectors by 638A-638C.The inventors have coined the term “rolled k-vector” to describe thevarying orientation of the k-vectors along the waveguide propagationdirection.

The principles illustrated in FIGS. 64-65 may also be applied in theoutput grating as illustrated in FIG. 66 . Here the output gratingcomprises a multiplicity of adjacently disposed gratings such as639A-639C with k-vectors 640A-640C. In alternative embodiments of theinvention the output grating may be comprised of stacked gratings basedon the principles of FIG. 64 .

A method of displaying an image is one embodiment of the invention inaccordance with the basic principles of the invention is shown in theflow diagram in FIG. 67 . Referring to the flow diagram, the method maycomprise the following steps:

At step 650 providing: a first optical substrate for propagating lightin a first direction; a second optical substrate for propagating lightin a second direction; and an Input Image Node (IIN), the first andsecond optical substrates comprising at least one waveguide layer, eachat last one waveguide layer comprising at least one grating lamina, andthe at least one grating lamina comprising a passive mode SBG.

At step 651 providing image modulated light using the IIN.

At step 652 coupling the image light into the First Optical Substrate;

At step 653 extracting light from first optical substrate along thefirst direction;

At step 654 coupling image light into the second optical substrate;

At step 655 extracting light from the first optical Substrate along thesecond direction;

At step 656 providing image light for display.

Summary of Some Embodiments

At least some embodiments provided herein use separate vertical andhorizontal beam expansion waveguides to provide an enlarged exit pupil(or eye box). Each waveguide contains input and output Bragg gratings.Each of the waveguides may comprise more than one waveguide layer. Incolor embodiments a separate monochromatic waveguide may be used foreach primary color. Another option for providing color is to recordmultiplexed gratings, in which holograms with different colorprescriptions are superimposed, into a waveguide.

Collimated image light is fed into the horizontal beam expansionwaveguide with a Field of View (FOV) defined by the microdisplay andcollimating optics. The invention allows the input or “coupling” opticsto be configured in many different ways ranging from classical opticallens-mirror designs to more compact designs based entirely ondiffractive (holographic) optics.

The horizontal beam expansion waveguide is lossy, that is, it isdesigned to extract light out of the waveguide uniformly along itslength. The extracted light is then coupled into the vertical expansionwaveguide.

The vertical expansion waveguide, which is also lossy, completes thebeam expansion to provide a vertically and horizontally expanded exitpupil.

A unique feature of the invention is that all of the above can beaccomplished using passive gratings (although the use of switchablegratings is still an option for some applications). Conventional passivegratings would not work. The chief benefit of using passive SBGs is thatthe refractive index modulation of the grating can be tuned from verylow to very high values with a correspondingly broad range ofdiffraction efficiencies. The high index modulation of SBGs results fromthe alternating bands of polymer-rich and LC-rich regions that form theBragg fringes.

While lossy gratings are known in the prior art, the present inventionis unique in achieving efficient and uniform extraction from thewaveguide by varying the thickness (and modulation) across the grating.In its simplest case this entails creating a wedged grating (byinclining the cell walls) such that the hologram thickness increases inthe direction of propagation. Typically, the grating thickness may varyfrom 1.0-1.2 micron up to 2.8-3.0 micron, the lower thickness producingthe lowest efficiency (and largest angular bandwidth). The inventionallows more sophisticated control of extraction by varying the thicknessin orthogonal directions, using two wedge angles, or in a more generalfashion by applying curvature to one or both faces of the grating.

A further unique feature of the beam expansion gratings is that they canbe made very thin (well below 3 microns) which results in very broaddiffraction efficiency angular bandwidth which, in turn, results in awide FOV. By optimizing thickness and refractive index modulation it ispossible to meet all of the needed grating characteristics needed in thedisplay, i.e., very high efficiency for coupling into gratings and largedynamic range for the efficient, uniform extraction needed for beamexpansion.

An extremely important feature of the invention that has implicationsfor image transfer inefficiency and form factor is the use of imagesampling. Coupling wide FOV image light into a waveguide would normallyresult in some loss of image angular content owing to the limited rangeof angles that can be efficiently propagated down a waveguide. Some ofthis light may couple out of the waveguide. The invention overcomes thisproblem by sampling the input image into multiple angular intervals,each of which has an effective exit pupil that is a fraction of the sizeof the full pupil, the thickness of the waveguide being reducedcorrespondingly.

Uniquely, the invention combines fixed frequency surface gratings at theinput and output of each waveguide with rolled k-vectors along thewaveguide. The surface grating is the intersection of the Bragg fringeswith the substrate edge and accounts (approximately) for the basic rayoptics of the waveguide. The k-vector is the direction normal to theBragg grating and accounts for the diffraction efficiency vs. anglecharacteristics of the grating. By varying the k-vector direction alongthe waveguide propagation direction (k-vector rolling) it is possibleto, firstly, provide efficient coupling of image light into thewaveguide and, secondly, ensure that once coupled-in, all of the neededangular content is transmitted down the waveguide with high efficiency.The k-vector rolling would desirably be augmented by grating thicknesscontrol as discussed above. To our knowledge this principle has not beenapplied in the prior art.

With regard to color imaging, making the input and output gratings ineach waveguide have the same surface gratings frequencies as discussedabove allows colors to be implemented in separate waveguides that arefree from cross talk. This is believed to be a unique feature of theinvention.

In general, the propagate of angular content down the waveguide can beoptimized by fine tuning of one or more of the following: gratingthickness; refractive index modulation; k-vector rolling; surfacegrating period; and the hologram-substrate index difference.

The apparatuses and methods described herein may be applied to HMD, HUDand HDD.

Exemplary Embodiments

The various aspects of the apparatus, systems, and methods describedherein may be further described in the various embodiments providedbelow:

In one embodiment the first optical substrate selectively samplesportions of the image modulated light, each portion being characterizedby either angular field or spatial field.

In one embodiment at least one grating lamina in each optical substratecomprises an input grating operative to diffract light coupled into saidsubstrate into a TIR path and an output grating operative to diffractlight from said TIR path out of said substrate.

In one embodiment extraction from said second substrate takes placethrough a face of the waveguiding layer.

In one embodiment extraction from second substrate takes place through awave guiding layer edge.

In one embodiment the grating vectors of grating lamina in the firstsubstrate lie in a plane substantially orthogonal to the faces of thefirst substrate.

In one embodiment the grating vectors of grating lamina in the firstsubstrate lie in a plane substantially parallel to the faces of thefirst substrate.

In one embodiment the waveguide layers are transparent dielectrics.

In one embodiment the waveguiding layers propagate monochromatic light.

In one embodiment first, second and third waveguiding layers areprovided in at least one of the first or second substrates for thepurpose of propagating red, green and blue light.

In one embodiment first and second waveguiding layers are provided in atleast one of the first or second substrates for the purpose ofpropagating red light and mixed blue and green light.

In one embodiment waveguiding layers in at least one of the first orsecond substrates sandwich a half wave film.

In one embodiment waveguiding layers in at least one of the first orsecond substrates sandwich an air space.

In one embodiment grating lamina in at least one of the first or secondsubstrates multiplex gratings of at least two different monochromaticprescriptions.

In one embodiment grating lamina in at least one of said first or secondsubstrates multiplex gratings of at least two different colors.

In one embodiment the first substrate provides pupil expansion along thefirst direction and the second substrate provides pupil expansion alongthe second direction.

In one embodiment light extracted from the first and second substratesprovides uniform illumination in any field of view direction.

In one embodiment each grating in at least one of the first substrate orsecond substrates has first and second diffracting state. The firstdiffracting state is characterized by high diffraction efficiency andthe second diffraction state is characterized by low diffractionefficiency.

In one embodiment the diffracting state occurs when an electric field isapplied across the grating and a non-diffracting state exists when noelectric field is applied.

In one embodiment the non diffracting state occurs when an electricfield is applied across the grating and a diffracting state exists whenno electric field is applied.

In one embodiment the first and second propagation directions areorthogonal.

In one embodiment at least one of the substrates is curved in at leastone orthogonal plane.

In one embodiment at least one of the waveguiding layers includes a beamsplitter lamina.

In one embodiment quarter wavelength film is applied to at least oneface of the waveguiding layer in either the first substrate or thesecond substrate.

In one embodiment a reflective thin film coating is applied to at leastone face of the waveguiding layer in either the first substrate or thesecond substrate.

In one embodiment the first coupling means comprises at least onegrating lamina substantially overlapping a portion of the firstsubstrate.

In one embodiment the first coupling means comprises at least onegrating lamina disposed within the waveguiding layer.

In one embodiment the first coupling means comprises at least onegrating lamina. Each grating lamina comprises at least two multiplexedgratings of different prescriptions. Each grating lamina substantiallyoverlaps a portion of the first substrate.

In one embodiment the second coupling means comprises at least onegrating lamina substantially overlapping the first substrate.

In one embodiment the second coupling means comprises at least onegrating lamina. Each grating lamina comprises at least two multiplexedgratings of two different prescriptions. Each grating laminasubstantially overlaps the first substrate.

In one embodiment the second coupling means is disposed within thesecond substrate.

In one embodiment grating vectors of grating lamina in the firstsubstrate lie in a plane substantially parallel to the faces of thesubstrate and the first substrate grating provides the second couplingmeans.

The invention may be used to provide one eye piece of a HMD, a HMD or aHUD.

In one embodiment at least one grating in the first or second substratesencodes optical power.

In one embodiment the second substrate is embedded within a windscreen.

Grating pitches covering the range of interest for practical displaysmay be achieved without difficulty; in fact, in one embodiment thematerial may sustain pitches as low as 0.2 microns and as high as 15microns. In electro-optical terms, POLICRYPS may be similar toconventional HPDLC. In some instances, the switching speed of POLICRYPSmay be higher than HPDLC and the switching voltage is at least equal toor lower than that of HPDLC. In one embodiment, like HPDLC gratingsPOLICRYPS grating may be utilized both in transmission and in reflectionand may be implemented in waveguides. The holographic recording processin POLICRYPS may be the same as that in HPDLC and may involve standardcommercially-available monomers and LCs. In one embodiment, onechallenge of POLICRYPS is that a high temperature process may be needed.In this embodiment, the temperature should be high enough to maintainisotropic mixture and to prevent isotropic-to-nematic transition duringexposure.

POLYICRYPS was developed at LICRYL (Liquid Crystals Laboratory,IPCF-CNR), Center of Excellence and Department of Physics, University ofCalabria, Italy). An example of POLICRYPS is described in Caputo, R. etal., Journal of Display Technology, Vol. 2, No. 1, March 2006, pp.38-50, which is incorporated by reference in its entirety. Furtherdetails of POLICRYIPS may be found in U.S. Patent Application No.2007/0019152 by Caputo. R.; et al., entitled “Holographic DiffractionGrating, Process for its Preparation and Opto-Electronic DeviceIncorporating It”; published in Jan. 25, 2007, which is incorporated byreference in its entirety

Another uniform morphology grating technology that may be employed isPOlymer LIquid Crystal Polymer Holograms Electrically Manageable(“POLIPHEM”), which was developed by the Fraunhoffer Institute forApplied Polymer Research, Potsdam (Germany). In one embodiment, POLIPHEMis similar to POLICRYPS in basic morphological and electro-opticalterms. One advantage of using POLIPHEM over POLICRYPS is that former mayavoid the high temperature processing needed in POLICRYPS in someinstances by optimizing the properties and proportions of LC and monomerin the material recipe. Details of materials and methods for fabricatingPOLIPHEM gratings may be found in the international patent publicationNo.: WO2006002870 (PCT/EP2005/006950) by Stumpe, J. et al., entitled“Method for the Preparation of High Efficient, Tunable and SwitchableOptical Elements Based on Polymer-Liquid Crystal Composites”, publishedJanuary 2006, which is incorporated by reference in its entirety.

In one embodiment, it may be desirable for the gratings described hereto provide both high diffraction efficiency and wide angular bandwidth.However, in one embodiment these two goals conflict in that wide angularbandwidth dictates that the gratings should be thin, while thin gratingsmay suffer from progressively diminishing diffraction efficiency as thethickness is reduced. One solution is to stack a multiplicity of thingratings, such that each grating may diffract the 0-order light from thegrating beneath it, so that most of the input light eventually may getdiffracted. In some embodiments, the grating layers may be separated byspacers. Such a stratified grating structure may be considered to beequivalent to diffraction by a thick or volume (i.e., Bragg) gratingwith at least one benefit of a much wider bandwidth resulting from usingthin gratings as a basic building block.

In one embodiment, the thin gratings may desirably operate in the Braggregime (rather than thin gratings according to the Raman Nath regime) toavoid higher order diffraction. In one embodiment, when the thingratings operate in the Raman-Nath regime, careful optimization of thethickness and pitch may be employed to ensure that the relative phasingof the diffraction orders as they propagate from layer to layer giverise to a unique notched diffraction response of the +1 order (for thecase of Bragg incidence). In another embodiments, Bragg gratings may berecorded in a stratified grating structure known as a Stratified VolumeHolographic Element (SVHOE). An example of SVHOE is described in Nordin,G., et al., J. Opt. Soc. Am. A., Vol. 9, NO. 12, December 1992, pp.2206-2217, which is incorporated by reference in its entirety.

A more complete understanding of the invention can be obtained byconsidering the following detailed description in conjunction with theaccompanying drawings, wherein like index numerals indicate like parts.For purposes of clarity, details relating to technical material that isknown in the technical fields related to the invention have not beendescribed in detail.

Additional Embodiments

The following embodiments are taken from the claims of the provisionalapplication Ser. No. 61/849,853, filed Feb. 4, 2013, which isincorporated by reference in its entirety.

-   -   1. An apparatus for displaying an image comprising:    -   an input image node for providing image modulated light;    -   a first optical substrate comprising at least one waveguiding        layer, each said waveguiding layer propagating light in a first        direction, each said waveguiding layer comprising at least one        grating lamina operative to extract light from said first        substrate along said first direction;    -   a second optical substrate comprising at least one waveguiding        layer, each said waveguiding layer propagating light in a second        direction, each said waveguiding layer containing at least one        grating lamina operative to extract light for display from said        second substrate along said second direction;    -   a first optical means for coupling said image modulated light        into said first substrate; and    -   a second optical means for coupling light extracted from said        first substrate into said second substrate.    -   2. The apparatus of embodiment 1 wherein said first optical        substrate selectively samples portions of said image modulated        light, each said portion being characterized by either angular        field or spatial field.    -   3. The apparatus of embodiment 1 wherein said at least one        grating lamina in each said optical substrate comprises an input        grating operative to diffract light coupled into said substrate        into a TIR path and an output grating operative to diffract        light from said TIR path out of said substrate.    -   4. The apparatus of embodiment 1 wherein said extraction from        said second substrate takes place through a waveguiding layer        face.    -   5. The apparatus of embodiment 1 wherein said extraction is        carried out from second substrate through a waveguiding layer        edge.    -   6. The apparatus of embodiment 1 wherein grating vectors of        grating lamina in said first substrate lie in a plane        substantially orthogonal to the faces of said substrate.    -   7. The apparatus of embodiment 1 wherein grating vectors of        grating lamina in said first substrate lie in a plane        substantially parallel to the faces of said substrate.    -   8. The apparatus of embodiment 1 wherein said waveguide layers        are transparent dielectric.    -   9. The apparatus of embodiment 1 wherein said waveguiding layers        propagate monochromatic light.    -   10. The apparatus of embodiment 1 wherein first, second and        third waveguiding layers are provided in at least one of said        substrates for propagating red, green and blue.    -   11. The apparatus of embodiment 1 wherein first and second        waveguiding layers are provided in at least one of said        substrates for propagating red light and mixed blue and green        light.    -   12. The apparatus of embodiment 1 wherein waveguiding layers in        at least one of said first or second substrates sandwich a half        wave film.    -   13. The apparatus of embodiment 1 wherein waveguiding layers in        at least one of said first or second substrates sandwiches an        air space.    -   14. The apparatus of embodiment 1 wherein grating lamina in at        least one of said first and second substrates comprises        multiplex gratings of at least two different monochromatic        prescriptions.    -   15. The apparatus of embodiment 1 wherein grating lamina in at        least one of said first and second substrates comprise multiplex        gratings of at least two different colors.    -   16. The apparatus of embodiment 1 wherein said first substrate        provides pupil expansion along said first direction and said        second substrate provides pupil expansion along said second        direction.    -   17. The apparatus of embodiment 1 wherein said light extracted        from said first and second substrates provides uniform        illumination in any field of view direction.    -   18. The apparatus of embodiment 1 wherein each said grating in        at least one of said first substrate or second substrate has a        first diffracting state wherein said first diffracting state is        characterized by a high diffraction efficiency and said second        diffraction state is characterized by a low diffraction        efficiency.    -   19. The apparatus of embodiment 17 wherein said diffracting        state occurs when an electric field is applied across said        grating and a non diffracting state exists when no electric        field is applied.    -   20. The apparatus of embodiment 17 wherein said non diffracting        state occurs when an electric field is applied across said        grating and a diffracting state exists when no electric field is        applied.    -   21. The apparatus of embodiment 1 wherein said first and second        propagation directions are orthogonal.    -   22. The apparatus of embodiment 1 wherein at least one of said        substrates is curved in at least one orthogonal plane.    -   23. The apparatus of embodiment 1 wherein at least one of said        waveguiding layers includes a beam splitter lamina.    -   24. The apparatus of embodiment 1 wherein quarter wavelength        film is applied to at least one face of said waveguiding layer        in either said first substrate or said second substrate.    -   25. The apparatus of embodiment 1 wherein a reflective thin film        coating is applied to at least one face of said waveguiding        layer in either said first substrate or said second substrate.    -   26. The apparatus of embodiment 1 wherein said first coupling        means comprises at least one grating lamina substantially        overlapping a portion of said first substrate.    -   27. The apparatus of embodiment 1 wherein said first coupling        means comprises at least one grating lamina disposed within said        waveguiding layer.    -   28. The apparatus of embodiment 1 wherein said first coupling        means comprises at least one grating lamina, each said grating        lamina comprising at least two multiplexed gratings of two        different prescriptions, each said grating lamina substantially        overlapping a portion of said first substrate.    -   29. The apparatus of embodiment 1 wherein said second coupling        means comprises at least one grating lamina substantially        overlapping said first substrate.    -   30. The apparatus of embodiment 1 wherein said second coupling        means comprises at least one grating lamina, each said grating        lamina comprises at least two multiplexed gratings of two        different prescriptions, each said grating lamina substantially        overlapping said first substrate.    -   31. The apparatus of embodiment 1 wherein said second coupling        means is disposed within said second substrate.    -   32. The apparatus of embodiment 1 wherein grating vectors of        grating lamina in said first substrate lie in a plane        substantially parallel to the faces of said substrate, wherein        said first substrate grating provides said second coupling        means.    -   33. The apparatus of embodiment 1 wherein said apparatus        provides one eye piece of a HMD, a HHD or a HUD.    -   34. The apparatus of embodiment 1 wherein at least one grating        in said substrates encodes optical power.    -   35. The apparatus of embodiment 1 wherein said second substrate        is embedded within a windscreen.    -   36. The apparatus of embodiment 1 wherein said wave guiding        layers have at least one face in contact with a nanoporous film.    -   37. The apparatus of embodiment 1 further comprising an eye        tracker.    -   38. The apparatus of embodiment 1 which further comprises a beam        homogenizer.    -   39. The apparatus of embodiment 1 wherein said input image node        comprises a microdisplay, laser and collimating optics.    -   40. The apparatus of embodiment 1 wherein said grating lamina in        at least one of said first or second substrates is an SBG.    -   41. The apparatus of embodiment 1 wherein said grating lamina in        at least one of said first or second substrates are        non-switching Bragg gratings recorded in HPDLC material.    -   42. The apparatus of embodiment 1 wherein said grating lamina in        at least one of said first or second substrates are SBGs        recorded in a reverse mode material.

REFERENCES

The following patent applications are incorporated by reference hereinin their entireties:

-   U.S. Provisional Patent Application No. 61/687,436 with filing date    25 April 12 by the present inventors entitled WIDE ANGLE COLOUR HEAD    MOUNTED DISPLAY;-   U.S. Provisional Patent Application No. 61/689,907 with filing date    25 April 12 by the present inventors entitled HOLOGRAPHIC HEAD    MOUNTED DISPLAY WITH IMPROVED IMAGE UNIFORMITY;-   PCT Application No. US 2008/001909, with International Filing Date:    22 Jul. 2008, entitled LASER ILLUMINATION DEVICE;-   PCT Application No.: US 2006/043938, entitled METHOD AND APPARATUS    FOR PROVIDING A TRANSPARENT DISPLAY;-   PCT Application No.: PCT/GB2010/001982 entitled COMPACT EDGE    ILLUMINATED EYEGLASS DISPLAY;-   U.S. Provisional Patent Application No. 61/573,066 with tiling date    24 August 12 by the present inventors entitled IMPROVEMENTS TO    HOLOGRAPHIC POLYMER DISPERSED LIQUID CRYSTAL MATERIALS AND DEVICES;-   PCT Application No.: PCT/GB2010/002023 filed on 2 Nov. 2010 by the    present inventors entitled APPARATUS FOR REDUCING LASER SPECKLE;-   PCT Application No.: PCT/GB2010/000835 with International Filing    Date: 26 Apr. 2010 entitled COMPACT HOLOGRAPHIC EDGE ILLUMINATED    EYEGLASS DISPLAY ;-   U.S. Pat. No. 6,115,152 entitled HOLOGRAPHIC ILLUMINATION SYSTEM,    issued 5 Sep. 2000; and-   U.S. Provisional Patent Application No. 61/796,795 entitled COMPACT    LASER ILLUMINATOR INCORPORATING A DESPCKLER with filing date 20 Nov.    2012.

Additional Example

FIG. 68 is a ray trace of a monochromatic version of the design. FIG. 69shows the approximate dimensions of the IIN of FIG. 68 . FIG. 70provides unfolded views of the optical layout of FIG. 69 .

The IIN stop is formed by controlling profile of input illumination.There is currently no hard physical stop in the projection optics. Thebenefits of a projected stop are decreased waveguide thicknesses. A stopis projected midway up the HBE to minimize aperture diameter within theVBE, and hence minimizing the aperture width of the VBE to DigiLenswaveguide coupler, i.e., reducing the width of the 1^(st) axis expanderlimits the thickness of the 2^(nd) axis expansion optic. FIGS. 71A and71B illustrate the formation of a projected stop inside the HBE using asimplified thin lens representation of the microdisplay projectionoptics.

In one embodiment a graduated reflection profile underneath SBG layer isused to control (or assist) with grating DE variation along length(normally achieved in SBG grating using index modulation). This may beuseful in cases such as the HBE, where a low percentage of light is outcoupled in 1^(st) bounce, but a high percentage is coupled out at theother end of the expander.

An Embodiment Using a Striped HBE

In one embodiment the HBE comprises a coupling grating at its input endand alternating SBG stripes of two different prescriptions are inclinedat 45 degrees within the plane as shown in FIG. 72 . Although thestripes are shown as equi-spaced, their size and spacing may be variedfor better illumination and image sampling control. However the stripesshould not be made too narrow as this may impact the MTF. In general,the stripe geometry may need careful optimization as there are, forexample, rays from extremities of a stipe that may result in a phasedifference in the pupil. The input SBGs need a large angular bandwidthand a high efficiency while the DigiLens passive grating is lossy.Although the angular bandwidths of the gratings have pronouncedcenter-to-edge variations, the extraction from the passive gratings isfound to result in more light being diffracted from the center of a beamextracted, creating an effective inversion of the illumination profileat the end of the waveguide. This effect can be used to advantage inbalancing the overall illumination profile. FIG. 73 illustrates beampropagation from the IIN through a single layer of the DigiLens showingthe four changes in direction that occur along the path to the exitpupil. The optical path is labelled by numerals 1-7.

FOV, Eyebox and Eye Relief Geometry

The near eye geometry of the proposed helmet mounted display is shown inplan view in FIG. 74 , in side view in FIG. 75 , and in front view inFIG. 76 . The relationship of the DigiLens® aperture to the FOV, eyerelief and eye box is also shown in FIG. 77 . Note that the DigiLens®aperture will scale with eye relief.

Binocular Overlap

As shown in FIG. 78 and FIG. 79 , partial binocular overlap can beprovided using convergent or divergent optics. In either case binocularoverlap can provide up to 1.4×contrast improvement. Convergent overlapmay be better for avoiding binocular rivalry. As a very tough guide,mostly distance work may need only low overlap, while mostly close-upwork (typically at arms length) may need higher overlap. In general,published data on FOV vs. task performance is often anecdotal or limitedto small and specialized samples. Extrapolating data from oneapplication domain to another can be risky. There is no research thatexamines the FOV vs. task performance tradeoffs for a particular type ofdisplay. Consequently, the effects of eyebox, geometric distortion, formfactor etc., are not fully reflected in the literature. The eyes tend tostay in the saccadic eye movement range (from 0° up to approximately±10° to ±15°). Outside this range the head will tend to move tore-center the image. If no head tracker is provided all importantinformation must lie with the saccadic region. Some applications need abalance between adequate peripheral cue presentation and central imagequality. Some research suggests that if the binocular overlap fallsbelow around 20° binocular rivalry leading to effects such as luning(shadowing around the edges of the overlap regions) will start to becomea problem for a significant number of users. The HWD overlap, in therange 20-25, has been chosen to maximize the overall horizontal field ofview while minimizing the risk of binocular rivalry.

Inter Pupillary Distance (IPD)

The Inter-Pupillary Distance (IPD) target is to have no interpupillarydistance adjustment for the majority of the adult population (90%). Morethan 90% of the adult population has IPDs in the range of 57 mm to 70 mm(+/−6.5 mm range). The pupil position change due to eye roll +/−20° is+/−4.5 mm. The tolerance of helmet placement/visor slip (budget) is+/−6.4 mm. Alignment may be estimated using the formula:Alignment=√[(IPD90%+Eye Roll)2+Slip2]=√[(6.3 mm+4.5 mm)²+6.4mm³]=√+/−12.5=25.0 mm wide eye box. Note that for the 10% of thepopulation with IPDs outside of the range, full field of view vision isprovided. For optimal alignment only one side of the field of view willbe lost for one eye only. The other eye will still see the other side ofthe field, i.e., 90% of the population get 100% overlap. The remaining10% of the population (within 52 mm to 75 mm IPD) will get 100% of thefield of view with some overlap dependent on IPD and display alignment.In conclusion, a 25 mm wide eye box will permit 90% of the population tobe accommodated without any adjustment, assuming the above alignmentparameters, improved alignment tolerances of the visor/head gear to theeye will enable a reduction in the eye box dimensions if needed. Thiscan be later traded off against system brightness.

Low Index Materials

Efficient waveguiding needs the TIR beams to be confined between lowindex media. Air gaps are difficult to fabricate and maintain while therefractive indices of currently available low index materials such asMagnesium Fluoride (1.46) and Silicon Dioxide (1.39) are much too highto meet the light TIR angle constraints needed in full colorimplementations of the HMD. The proposed solution is to use nanoporous(Mesoporous Silicon) materials. Nanoporous materials (e.g., mesoporousSilicon) are currently being used in many optical applications includinganti reflection coatings and planar optical waveguides. Their highporosity enables the fabrication of high-quality low-dielectric constantthin films. Nanoporous materials can be fabricated in thin layers in asingle coating step. To achieve very low, near unity, index theporosities need to be very high, approaching 95%. High transparency andlow index can be achieved simultaneously with these films. Since theyare highly efficient at absorbing water, they must be carefully sealedagainst moisture. The best approach may be to seal the passive gratings,HWP and material together. SBG Labs is also investigating the potentialrole of nanoporous materials as high refractive index media. This wouldincrease the range of TIR angles that can be sustained in our waveguideswith potential for increasing the horizontal POV from 40° to around 45°.Nanoporous materials are currently being used in many opticalapplications including anti reflection coatings and planar opticalwaveguides. It is reasonable to assume therefore that the technologywill be accessible for our project. The manufacturing process should betranslatable to manufacturing needs. Nanoporous materials can befabricated in single coating step. Alternatively, graded indexmultilayer architectures can be used. SBG Labs is also investigating thepotential role of nanoporous materials as high refractive index media.This would increase the range of TIR angles that can be sustained in ourwaveguides.

CONCLUSION

All literature and similar material cited in this application,including, but not limited to, patents, patent applications, articles,books, treatises, and web pages, regardless of the format of suchliterature and similar materials, are expressly incorporated byreference in their entirely. In the event that one or more of theincorporated literature and similar materials differs from orcontradicts this application, including but not limited to definedterms, term usage, described techniques, or the like, this applicationcontrols.

While the present teachings have been described in conjunction withvarious embodiments and examples, it is not intended that the presentteachings be limited to such embodiments or examples. On the contrary,the present teachings encompass various alternatives, modifications, andequivalents, as will be appreciated by those of skill in the art.

While various inventive embodiments have been described and illustratedherein, those of ordinary skill in the art will readily envision avariety of other means and/or structures for performing the functionand/or obtaining the results and/or one or more of the advantagesdescribed herein, and each of such variations and/or modifications isdeemed to be within the scope of the inventive embodiments describedherein. More generally, those skilled in the art will readily appreciatethat all parameters, dimensions, materials, and configurations describedherein are meant to be exemplary and that the actual parameters,dimensions, materials, and/or configurations will depend upon thespecific application or applications for which the inventive teachingsis/are used. Those skilled in the art will recognize many equivalents tothe specific inventive embodiments described herein. It is, therefore,to be understood that the foregoing embodiments are presented by way ofexample only and that, within the scope of the appended claims andequivalents thereto, inventive embodiments may be practiced otherwisethan as specifically described and claimed. Inventive embodiments of thepresent disclosure are directed to each individual feature, system,article, material, kit, and/or method described herein. In addition, anycombination of two or more such features, systems, articles, materials,kits, and/or methods, if such features, systems, articles, materials,kits, and/or methods are not mutually inconsistent, is included withinthe inventive scope of the present disclosure.

Also, the technology described herein may be embodied as a method, ofwhich at least one example has been provided. The acts performed as parof the method may be ordered in any suitable way. Accordingly,embodiments may be constructed in which acts are performed in an orderdifferent than illustrated, which may include performing some actssimultaneously, even though shown as sequential acts in illustrativeembodiments.

All definitions, as defined and used herein, should be understood tocontrol over dictionary definitions, definitions in documentsincorporated by reference, and/or ordinary meanings of the definedterms.

The indefinite articles “a” and “an,” as used herein in thespecification and in the claims, unless clearly indicated to thecontrary, should be understood to mean “at least one.” Any ranges citedherein are inclusive.

The terms “substantially” and “about” used throughout this Specificationare used to describe and account for small fluctuations. For example,they can refer to less than or equal to ±5%, such as less than or equalto ±2%, such as less than or equal to ±1%, such as less than or equal to±0.5%, such as less than or equal to ±0.2%, such as less than or equalto ±0.1%, such as less than or equal to ±0.05%.

The phrase “and/or,” as used herein in the specification and in theclaims, should be understood to mean “either or both” of the elements soconjoined, i.e., elements that are conjunctively present in some casesand disjunctively present in other cases. Multiple elements listed with“and/or” should be construed in the same fashion i.e., “one or more” ofthe elements so conjoined. Other elements may optionally be presentother than the elements specifically identified by the “and/or” clause,whether related or unrelated to those elements specifically identified.Thus, as a non-limiting example, a reference to “A and/or B” when usedin conjunction with open-ended language such as “comprising” can refer,in one embodiment, to A only (optionally including elements other thanB); in another embodiment, to B only (optionally including elementsother than A); in yet another embodiment, to both A and a (optionallyincluding other elements); etc.

As used herein in the specification and in the claims, “or” should beunderstood to have the same meaning as “and/or” as defined above. Forexample, when separating items in a list, “or” or “and/or” shall beinterpreted as being inclusive, i.e., the inclusion of at least one, butalso including more than one, of a number or list of elements, and,optionally, additional unlisted items. Only terms clearly indicated tothe contrary, such as “only one of” or “exactly one of,” or, when usedin the claims, “consisting of,” will refer to the inclusion of exactlyone element of a number or list of elements. In general, the term “or”as used herein shall only be interpreted as indicating exclusivealternatives (i.e. “one or the other but not both”) when preceded byterms of exclusivity, such as “either,” “one of,” “only one of,” or“exactly one of.” “Consisting essentially of,” when used in the claims,shall have its ordinary meaning as used in the field of patent law.

As used herein in the specification and in the claims, the phrase “atleast one,” in reference to a list of one or more elements, should beunderstood to mean at least one element selected from any one or more ofthe elements in the list of elements, but not necessarily including atleast one of each and every element specifically listed within the listof elements and not excluding any combinations of elements in the listof elements. This definition also allows that elements may optionally bepresent other than the elements specifically identified within the listof elements to which the phrase “at least one” refers, whether relatedor unrelated to those elements specifically identified. Thus, as anon-limiting example, “at least one of A and B” (or, equivalently, “atleast one of A or B,” or, equivalently “at least one of A and/or B”) canrefer, in one embodiment, to at least one, optionally including morethan one, A, with no B present (and optionally including elements otherthan B); in another embodiment, to at least one, optionally includingmore than one, B, with no A present (and optionally including elementsother than A); in yet another embodiment, to at least one, optionallyincluding more than one, A, and at least one, optionally including morethan one, B (and optionally including other elements); etc.

In the claims, as well as in the specification above, all transitionalphrases such as “comprising,” “including,” “carrying,” “having,”“containing,” “involving,” “holding,” “composed of,” and the like are tobe understood to be open-ended, i.e., to mean including but not limitedto. Only the transitional phrases “consisting of” and “consistingessentially of” shall be closed or semi-closed transitional phrases,respectively, as set forth in the United States Patent Office Manual ofPatent Examining Procedures, Section 2111.03.

The claims should not be read as limited to the described order orelements unless stated to that effect. It should be understood thatvarious changes in form and detail may be made by one of ordinary skillin the art without departing from the spirit and scope of the appendedclaims. All embodiments that come within the spirit and scope of thefollowing claims and equivalents thereto are claimed.

What is claimed:
 1. An apparatus for displaying an image comprising: aninput image node providing image modulated light, the image modulatedlight including image content associated with at least a portion of adisplay field of view; a beam expander comprising: an expander opticalsubstrate; one or more first expander input Bragg gratings to couple theimage modulated light into one or more waveguide layers of the expanderoptical substrate for propagation along a first direction; and one ormore expander output Bragg gratings in the one or more waveguide layersof the expander optical substrate, wherein the one or more expanderoutput Bragg gratings extend along the first direction to progressivelyextract portions of the image modulated light out of an output face ofthe expander optical substrate, wherein the one or more expander outputBragg gratings have increasing diffraction efficiency along the firstdirection to provide uniform extraction of the image modulated light outof the output face of the first expander optical substrate along thefirst direction; a display substrate comprising: a display opticalsubstrate; one or more display input Bragg gratings in the displayoptical substrate configured to couple the image modulated light fromthe beam expander into one or more waveguide layers of the displayoptical substrate and to propagate the image modulated light along athird direction perpendicular to the first and second directions; andone or more display output passive-mode switchable Bragg gratings in theone or more waveguide layers of the display optical substrate, whereinthe one or more display output passive-mode switchable Bragg gratingsextend along the third direction to progressively extract portions ofthe image modulated light along the third direction out of an outputface of the display optical substrate, wherein the one or more displayoutput passive-mode switchable Bragg gratings have increasingdiffraction efficiency along the third direction to provide uniformextraction of the input light out of the output face of the displayoptical substrate along the third direction, wherein the one or moredisplay output passive-mode switchable Bragg gratings direct the imagemodulated light along angles associated with the at least a portion ofthe display field of view, wherein the image modulated light from theone or more display output passive-mode switchable Bragg gratings islinearly polarized.
 2. The apparatus of claim 1, wherein the one or moreexpander output Bragg gratings include alternating stripes of Bragggratings with different prescriptions.
 3. The apparatus of claim 2,wherein the alternating stripes of Bragg gratings are angled at 45degrees relative to the first or the second directions.
 4. The apparatusof claim 1, wherein at least one of the one or more expander outputBragg gratings or the one or more display output passive-mode switchableBragg gratings have varying thickness along the respective direction oflight propagation to provide for uniform extraction.
 5. The apparatus ofclaim 4, wherein at least one of the one or more expander input Bragggratings or the one or more expander output Bragg gratings include anactive switchable Bragg grating.
 6. The apparatus of claim 1, whereinthe first direction is opposite to the second direction.
 7. Theapparatus of claim 1, wherein the first direction the same as the seconddirection.
 8. The apparatus of claim 1, wherein at least one of the oneor more expander input Bragg gratings or the one or more expander outputBragg gratings include switchable Bragg gratings.
 9. The apparatus ofclaim 8, wherein the one or more expander input Bragg gratings and theone or more expander output Bragg gratings include passive-modeswitchable Bragg gratings.
 10. The apparatus of claim 9, wherein the oneor more expander input Bragg gratings and the one or more expanderoutput Bragg gratings include non-switching Bragg gratings recorded in aholographic polymer-dispersed liquid crystal (HPDLC) material in atleast one of forward or reverse mode.
 11. The apparatus of claim 1,wherein the image modulated light comprises monochrome light.
 12. Theapparatus of claim 1, wherein the first image modulated light comprisesmulti-wavelength light.
 13. The apparatus of claim 1, wherein the one ormore expander input Bragg gratings and the one or more expander outputBragg gratings are complementary, wherein the one or more display inputBragg gratings and the one or more display output passive-modeswitchable Bragg gratings are complementary.
 14. The apparatus of claim1, wherein each of the one or more waveguide layers of at least one ofthe beam expander or the display substrate is configured to propagate atleast one of red, green, blue, blue/green mixed light, and one of amultiplicity of sub field of Views (FOVs).
 15. The apparatus of claim 1,wherein at least one of the one or more waveguide layers of at least oneof the beam expander or the display substrate comprises holograms withsuperimposed different color prescriptions.
 16. The apparatus of claim1, wherein at least one of the one or more waveguide layers of at leastone of the beam expander or the display substrate is lossy.
 17. Theapparatus of claim 1, wherein of the one or more waveguide layers of atleast one of the beam expander or the display substrate has a varyingthickness along the respective direction of light propagation.
 18. Theapparatus of claim 1, wherein at least one of the beam expander or thedisplay substrate is curved.
 19. The apparatus of claim 1, wherein theimage modulated light from the one or more display output passive-modeswitchable Bragg gratings is uniform across the at least a portion ofthe display field of view.
 20. A device comprising the apparatus ofclaim 1, wherein the device is a part of at least one of a HelmetMounted Display (HMD) or a head up display (HUD).